What I think you have to bear in mind is that you have access to all the information, most of the characters within the show don't, especially with regards to the nature of the Dollars and their abilities. I suppose this is a version of
J.J. Abrams mystery box theory, the idea of which is that when you only have some of the information the possibilities of what it might be are always far more fantastical and exciting than the reality. Put simply: The mystery is always greater than the reveal. Nobody in the show has any idea what the Dollars are (arguably even Mikado), so the possibilities of what it might be are far more powerful, far more scary, far more violent, far more dangerous than the reality. And people fight over dangerous maybes all the time, the risk of being or becoming the ridiculous idea in their head is too big to take. They aren't really fighting over their reality, they're fighting over possibilities. (Which now that I think about it, might be the point.)
This isn't really relevant but I also think, especially with the second season, that Durarara!! shouldn't just be analysed on a narrative level, but also on how the audience reacts. Maybe I'm interpreting things weirdly, but to me Durarara!! is more about the way people and society (both inside and outside the show) react to the behaviour of the gangs, rather than the way the gangs behaved. I assume based on your tweets you intend to start DRR!!x2 soon? I'd be interested to know what, if anything, you (or anybody else) makes of it on a philosophical level.
I feel like I was maybe reading stuff into Durarara!! that wasn't there, but simultaneously Narita seems smart enough to maybe be aiming for something deeper than the surface level appeal of the series. Maybe I'll try and schedule a rewatch when AL finally finished home video for it. (Presumably early 2018.)