Damn, I hope I'm not too late to this party because if you see my comment history, I just preach about this show in almost every /r/anime thread. Anyways...

Yeah, everything might work out great if she wasn't there, but that's not the point of the show. I don't think Nana is supposed to have a happy ending like all the other romcoms out there and it was never intended to. It's supposed to make you feel mad.

Here's where I think Nana K.'s character, though infuriating, is the most essential piece to the show (besides the obvious fact that she's the main character). It shows that people aren't perfect and make really dumb mistakes that fuck up their and other peoples' lives forever. A lot of times, especially in shoujo or other girl-oriented romance anime, the main female protagonist is like this idealized version of a girl. She's pure and innocent, good-hearted, very pretty but humble about it (or plain-looking but has such a good heart that it MAKES her pretty to others), doesn't even realize that though she's dating the school idol that her boyfriend is perfect, etc. And as such, most of the conflicts in these shows are like brought on by some external force- an evil temptress who latches onto the protag's boyfriend and tries to make life for the protag a living hell, a misunderstanding like the protag thinking she saw her boyfriend on a date with another girl (but it actually turns out to be his yet to be introduced sister or something), or the parents are against their relationship. Then, because the problems are external, they usually get solved within one "arc" of the story, and never arise again (with the occasional throwback to it until the final resolution). It's like Hercules' trials to become a god or something, they each present a problem, it's dealt with in succession, then happily ever after.

But in Nana, Nana K. fucks up her own romantic relationships, life, and happiness. It's not some divine power intervening and causing all these hardships that test the main couple's love. She also causes inconveniences to those around her. She's stupid, selfish, both vain and insecure at the same time, shallow, rash, full of regrets, and acts accordingly. And though her problems are exacerbated due to "external" influences sometimes (ex. Nobu is poor, Takumi is rich, Nana O doesn't have time to see her b/c she's getting more famous, etc.), it comes down to just her as a person being the problem. The way she thinks and attempts to solve things is the real conflict in the series. No, it's not that Takumi is just a gigantic asshole, no it's not because she had a surprise pregnancy, it's how SHE deals with it that's the big underlying problem. And it keeps piling up because she never solves the INTERNAL problems that are the underlying cause of all this conflict in her life. She tries to solve the external ones as best as she can, but everything keeps piling up because she hasn't attacked the root of the problem, which is herself. So yeah, like you said, everything might work out with a different person at the helm of the ship of these problems.

But what 18 to 20-something year old girl isn't one of these things at some point (vain, shallow, insecure, stupid, etc.)? Any girl (or boy for that matter) who has gone through this awkward young adult phase, stuck between in that time when you are just leaving home and are trying to be a "real adult" would be a delusional "special snowflake" if they told me they weren't one of these things.

As such, the actions she takes reflect choices that real people in this age bracket at least consider, and sometimes actually follow through and make. But it also shows their consequences. Oh boy, does Yazawa show their consequences. And it makes you feel the strong emotions that come along with the negative consequences. Sure, you might not be feeling the same heartbreak that Nana does when she hears Takumi calling Nobu from her room, but you're feeling something bad, aren't you? It's almost as if you're experiencing the repercussions yourself, and maybe it'll teach you to do it differently if you're ever in a similar situation. I'm not saying Yazawa was trying to be didactic in writing all these heart-wrenching and frustrating scenarios at all, but she really does a great job in evoking kind of the same family of emotions that the characters are feeling in the reader, so it's a more engaging experience and makes it a bit more human and realistic. It also presents a show where problems aren't solved in every arc and just pile up if you don't attack the root cause, which often times is yourself. And okay, you don't physically "attack" yourself, but I would hope that as people go through their lives in the real world, they try and better themselves. More than the romance and friendship, I think Nana is about showing these two young women who are going through this process, slow, very "one step forward, two steps back," and frustrating, as it may be, to better themselves and become "real adults."

There's my 2 cents on Nana (though it's really like $20 with that wall of text...sorry about that) It wasn't all spurred by this specific comment, but I just picked one that kind of dealt with my general thoughts on the show and replied to it, so lucky you ;P I just have a lot of feelings about this show.