I think we'll see a bunch of the top communities gravitate towards a few instances, but the userbase will spread out over time. Right now people are going to the biggest, b/c they just want to get on and try it out. But over time as people learn how to work with this new system they'll venture out to others. I could see a lot of people eventually hosting their own instance just for their one user, but have no communities of its own.
But for communities, finding an instance that has the right rules and plugins available will lead them to looking for trusted servers to moderate them from. As long as what Beehaw's doing doesn't become a trend, it should be fine