You asked for an opinion and this is the monster I give you in return. Good luck, my friend.

First, I’m not a popular 1d blog, if by ‘popular’ you mean that I get a lot of asks lol no I’m definitely not. If popular means ‘popular by tumblr standards’, I don’t even know what that means and, should we really apply sick popularity rankings and social hierarchies on Tumblr too? (I’m a pain in the ass sometimes and I know it)  
Sorry I didn’t mean to be aggressive or polemic, I just don’t like the term popular, but I get what you mean and I like your question =) 
Anyway, back to your question, my answer is very predictable.  
You might have noticed how much I tend toward the Louis-girl side of this fandom, also, I’ve already kind of talked about this before… so yeah, shocker, I think Louis’ departure would affect the dynamic of the band the most.
My apologies, this is post is about to become an eulogy of Louis.
The boys are all important, of course they are, but I think Louis had a key role in the construction of the band and in the construction of the band’s image. So I think he’s essential for the internal dynamic of the band but also for the external appearance of it, they can find ways to fabricate the same spontaneousness they have with Louis in it, but it wouldn’t be the same without him. It would feel as a mare facade, it would feel empty. 
All boybands have distinctive characteristics and a precise target demographic, and One Direction have too.  
I see the boys as a very Monkeyesque kind of band with a very young fanbase. 
This is their thing and this is their demographic, it’s easy to understand many of their marketing moves if you keep this idea in mind. 
They need to be funny and desexualized. That’s One Direction™. 
They don’t use sex to attract girls. Their handlers try to keep them as much desexualized as they can.  
Regardless of what the press says, and of their very cheeky-yet-vanilla lyrics, they still have a very clean image. They are good guys.  
I like to say that they are highly sexualized desexualized sexual objects.  
They need to attract girls (hence they are sexual objects) but not fully sexually developed girls (hence they are desexualized and the farthest thing from controversial). They need to be husbands, romantic dreams, material not sexual partners, steamy dreams, material. 
So sex is not their thing.  
The 90s boyband trope is not their thing either. 
They don’t dance and don’t dress all the same and they also have a slightly younger audience than their precursors (they don’t try to appeal to an older audience, say 20s/plus girls, and that’s very, very sad since I’m one of the ‘old fanladies’).  
No sex, no tropey things (not as many anyway) so they needed something else.  
They needed something to be different, to shine between many other wannabe-world famous-boybands, and they used two of their natural characteristics, what caught people’s attention since their xfactor days:  

  • Their sense of humor. Their funny, carefree, generally crazy attitude.  
  • The very obvious brotherly love they feel for each other and how public and relaxed they are with it.

All this introduction to come here… those are two strong characteristics of Louis’ own personality as well, that’s why I believe Louis had a strong role in the shaping of their image.  
They are funny. They’ve always been funny, mostly in their diaries and most of the time because Louis acted crazy and the boys followed suit. But they weren’t funny on the stage. Now they are, and they are funny in their official videos too. They took that direction with all the videos from Take Me Home because their sense of humor is their strong point.  
After Xfactor, their diaries and behind the scenes and silly game shows and interviews and stuff all consolidated this image of funny, immature, crazy boys and, if you look at it, Louis always was, and still is, the master of ceremonies behind all of them. No Louis, no funny spontaneous 1D, not as much at least. 
But I’m not talking about the effect it would have on the band, am I? Let me fix this by talking about Louis some more, because I love talking about Louis. 
I think that Louis is clearly the unspoken leader of the band, all the boys look up to him and all the boys got more and more comfortable acting stupid because Louis acted stupid first. (This is how many groups of friends work. They need someone like Louis, he puts everyone in a comfortable position because no one can judge you badly for being silly if there’s someone else in the group that acts sillier than you, and feels completely comfortable about it. If that someone is also the leader, that’s even better. You can be yourself without feeling judged and the group shields you from the opinions of the outer world as well). So, yeah, I think this is a very important function that only Louis can cover in the band and that the boys would miss if he wasn’t with them anymore.  
In a similar way, I think Louis was the medium though which the group came together. Now they are all ‘brothers’ but if you look back you can see that it wasn’t like that at first. All the boys were friends but it was almost as if they were all closer to Louis than to everyone else, Louis was the core of the group, they were all bound to each other because they were all bound to Louis, he kind of brought them together. All individually friends with Louis first, then all friends with each other. It’s a bit more complex than this but I’m trying to keep it short. 
I remember talking anonymously of this with lapelosa a few months back … anyway, I think this non-heteronormative, free from rigid borders attitude they have, started from Louis as well. And, in a way, from Harry and Louis very peculiar relationship.  
Louis has always been the most touchy-feely. You can’t live for long with Louis without being infected by his touchiness, you have to get accustomed to it, you need to be comfortable with it or it gets impossible to live, and all the boys got slowly accustomed to it, by choice or by need. So much so that they became the super touchy band we now know. 
I drawn this conclusion from Liam’s relationship with Louis, and now I’ll tell you all about it…  
Harry was clearly more than okay with Louis’ touchiness since the very beginning (of course he was they were already banging). For Niall and Zayn was very easy to get used to it and start acting accordingly ‘cause in different ways they have very open personalities. 
For Liam it wasn’t as easy, or at least it didn’t seem as easy.  
In my opinion it’s very apparent that Louis helped him coming out of a slightly passive homophobic state. He still falls back in it sometimes but I can forgive him because there’s no harm intended in his actions.  
I don’t want to be mean toward Liam, this is not my intention, I really like him, I sincerely like him but I also like to be frank. He’s a good guy but he reminds me of many boys I’ve known in my life, these frat boys (I don’t even know if the term suits), a bit shallow (not necessarily in a bad way), only devoted to an easy, happy, fun life. Without a single care in their minds. The very typical boy. The most common type of teenager.  
Living like this, with a certain kind of friends (that in all probability molded you into the shape you are), makes you assimilate the very conformist conception of “what a man must be”. A subtle, passive, almost subconscious, homophobic mentality that comes with certain expectations and many uncrossable borders. And what’s the punishment for going against all these unwritten rules? Being called gay, the worst insult you can throw at anyone, you know? Gay, cunt, pussy, sissy, girl. A very homophobic and misogynistic concept that most boys learn from society and that they accept blindly only to be accepted by the pack. 
Anyway I think Liam came from one of these places and it was hard for him to open up and let Louis in, and god knows how much Louis was knocking and fisting at that damn door with all his strength. Louis presses and presses and presses until he wins you over, and eventually Liam gave up and slowly opened up.  
I really believe Louis helped him become more accepting and less scared of the judgment of society, and at some point Liam started being more comfortable with the constant invasion of his personal space and with being touchy himself, at first only with Lou, then with all the boys too; but with Louis is still different, he feels entitled to take it further with him, he feels more comfortable. I think it’s because Louis sparked the change and because Liam feels totally accepted by him. So comfortable, actually, that he crosses too many lines, and sometimes Louis pushes him off. Borders are not as rigid as you thought they were, dear Liam, but they still exist, and some of them you can’t still cross. It seems like he doesn’t understand that well, he’s still a bit clumsy with it. But we can fogive him because he’s new to it, after all.  
This is getting really long and rambly and it’s taking the form of an ‘ode on the flawless Louis Tomlinson’; but there are reasons why I like Louis so much, I just mentioned some, and some I will say now.  
Louis really is the emotional leader of this band, and he was the glue that kept them together, and he helped in the process of construction of a recognizable, distinct image; but Louis is also a practical leader. Even the boys said that. 
It’s clear to me that he knows very well how to play the game of fame. He’s not a good liar, no, but he knows how this industry works and he’s very cynic about it (I think Zayn is as well and he doesn’t get enough credit for it), he knows  what people want, what he should give them. It’s part of being a natural entertainer, and he is a natural entertainer. 
He pretends to be immature and random but that’s a role, many things he does are very calculated, his spontaneousness is very calculated as well, he knows he can play with it because people loves it.  
I’ve read that people from their staff said that he’s what in football you’d call a ‘coach on the field’, a manager on the field, and it’s not hard for me to believe that. There’s a lot more thought behind his actions than people give him credit for.  
He’s also very protective and supportive of all the boys. Maybe ‘cause he’s the oldest, maybe ‘cause he helped his mother rising his little sisters, I don’t know, but he’s always at the forefront when people offend his boys, and that’s another reason why I think he’s indispensable for the well-being of the band. 
To end this long-ass Louis’ eulogy, my answer is: Louis because he’s the true leader of the band. He brings spirit, union, protection, support and a direction. I should have given you this answer, the short answer, straight away but I’m very verbose and I can help it but ramble and ramble and ramble. 
This is what I think about the inner dynamic but I feel that on the ‘success level’ if any of them ever left the band their management would find ways to compensate the absence, they would probably lose some of their following but the boys would survive, and keep being a pretty successful band. I believe that only Harry’s departure would affect their success, it would have a massive impact on the media, and their career would probably go downhill.

P.s. I’m curious, what are the other blogs’ responses?

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