This is a concern, but luckily this isn’t required. I set up hobbit.world to host my Tolkien related communities. It only costs $6 a month plus the $35/yr for the domain name to host a tiny instance like this. I don’t need to depend on anyone but my hosting provider.
To be safe I should download backups once a month or so.
But the point is that for big communities that people put a lot of time into, there should be an instance for each one owned by one of the mods.
Edit: Meant to reply to the person concerned about the centralization of communities.