This sucks, and might be the reason I return to Reddit. This pretty much splits the Lemmy population in two halves
Not really, I'm sure there are other Lemmy instances that have access to both. I'm on kbin for example and can see and interact with lemmy.world, beehaw.org, and sh.itjust.works.
Yeah, but at the moment, beehaw, ml, world and shitjustworks are the 4 biggest lemmy servers. Now, beehaw and ml will stay on the beehaw communities, while world and shotjustworks will have to split out into new ones
Or you can have an account on other than the 4 big servers and still be able to see and interact with all content.
This only solves the problem for you. It doesn't solve the problem for the community, nor does it scale well (if everyone does it, then you're just gonna repeat the same dance over and over).

I mean if the users of one big instance would be on 100 smaller ones, it would provide a more granular ability to defederate.

I'm only pointing out that the whole idea of the Fediverse would be to register wherever, where you feel "at home" with like-minded users instead of just making an account at the biggest server, as that makes managing the network untenable.

I'm just saying this is one of the reasons there shouldn't be a "big 4" of instances. Especially as the Beehaw guys seem to be acting out their powermod fantasies.