yeah, i agree the issue of multiple communities serving the same purpose is minor and i don't expect it to be an issue as time goes on.
maybe it's not as much of an issue as i think but my concern would be if one instance has the vast sum of users would others be discouraged from defederating with it. if a benefit of being federated is being easily discoverable by users, than having the largest userbase would make federating with that instance inherently more valuable as communities would want to be found by those who would participate.
it could be that it wouldn't be a big issue to exist away from large instances, and i'm sure many communities wouldn't need or want to seek out users through large general instances. i just wouldn't want admins of large instances to hold unequal power over smaller federated instances that would want to reach the largest userbase.
no nazis is big, hope we can keep that up.