Also, don't forget that you don't belong to one community; it's not like you log in one day and get assigned to your immutable chunk of the internet. You wander around the read versions of various communities, pick a couple of communities you invest it, pick several more to half-follow, get tired of the abuse in one and switch to another, meet someone in real life who encourages you to join another community, and so on.
Basically, the sort of "tribalism" that you'd encounter is whatever "tribalism" you associate with HN, because the structure I describe already half-exists, scrounging around the remnants from reddit and Facebook in tens of thousands of forums and active blogs and other such things. I don't think it produces any more or less "tribalism" than you get from anything else.
Social media is like being in a room with thousands, possibly millions, of people. If you have a gathering of this many people in real life it's entirely possible for the crowds to go out of control and for them to turn into a mob. This mob then gets worse through one-upmanship.
Subdividing these groups might breed new tribalism, but it'll make it less likely for a situation to spiral out of control. It takes more time for information to pass the barrier between communities. You also won't get as many people engaging in group-think, because your in-group isn't as large.
Not directly; I'm increasingly thinking centralization is not the problem.
The problem is size.
The intersection of "things that everyone agrees are acceptable" goes to zero as the size of the community increases, and while communities have strong selection effects that prevent them from being anything like "a uniformly random selection of people slammed together", regression to the mean means that you get there as size goes into the tens of thousands and hundreds of thousands and millions.
I don't think there's a solution, because no matter what technical tools you put into place, you can't get around the fact the people don't agree about what's acceptable.
What we need is a diaspora back into a community structure where HN would be just about the largest sort of community out there, instead of still a relatively small one. Decentralized tools could help with that, but if "decentralization" just ends up building "decentralized" communities trying to contain a million people in the "same place" you'll just get the same problems again. It's kinda orthogonal. "Reddit, but once a community hits 10,000 people it becomes progressively harder to join" would quite possibly work fine, for instance. (I envision something like an exponentially-growing waiting list delay, such that getting to even 11,000 users would be difficult.)
Obviously, legislating some sort of limit here is virtually impossible. Somehow the technical structures need to support it and afford communities in the hundreds to low thousands, but somehow make it inconvenient to grow beyond that. Or afford more complicated structures where membership is not a binary status. (A social graph would work, if it wasn't being run by an entity that desires to aggregate people together more to more effectively advertise at them. Or more accurately, a set of graphs.)