Protecting a community from this is what the decentralized part is for. That is already in place.
What? How is it solved exactly? If say lemmy.ml is down, what's the point of other servers existing, if most of the content and users are here? Like, I created a few new communities on lemmy.ml, which don't exist on say Beehaw because for some strange reason, the Beehaw admins don't allow users to create communities. So how is going to Beehaw help me, if lemmy.ml is unavailable? Okay, so you tell me I should go to a different server then. Maybe even make a new server. Done and done. But there's very few to zero users on that server, so those new communities and content created there might as well not exist. Also, even though Lemmy is federated, the homepage defaults to "local", so all the new users coming in may miss out on all the other federated communities, and, if I'm reading this correctly, the federation isn't even a fully automatic process, and some admins may even choose to use it in a whitelist mode. All of it makes the whole "advantage" of federation, or at least Lemmy's version of it, seem kind of pointless.
It's like saying, "Hey, Gmail is down so you should just use Hotmail instead." Okay, so I can still send and receive emails, but I can't access any of my old emails for context, and none of my contacts can reach me using my Gmail address, and none of my filters, address book and other content is available so I may not even be able to reach out to my contacts and let them know what my new email is.
IMO the way the way the federation should've been designed is to use something like blockchain technology, so every instance basically has all the content and there's only one source of truth for user accounts and data (distributed ledger), or maybe even just implement the whole thing as a plain old high-availability cluster with load balancing.
Unless I'm missing something fundamental, I don't see how this decentralization is of any use if the content isn't there.