So if I understand this right, you pick a server and your server account can post on that server and any servers that that server federates with.
So what happens to your account if the server you joined goes down? Yes, you could always create a new account somewhere else, but you lose all your followed communities and post and comment histories as well.
I suppose, the more I think about it, the same is true right now, except instead of a single instance having a monopoly on a topic, its reddit. I totally agree though its a valid concern, and I think something like multi reddits is the answer. If I can just subscribe to all instances that are tagged under a certain topic, then all those other instances can host content and still get visibility, instead of being crushed by those mega instances.