What federation protects from is the singular owner of the platform sweeping in and setting/enforcing new rules for some or all communities. This could still happen on one instance, but new instances can mitigate the effects. Single communities can still turn bad, but it will be up to the users to decide whether to stick around or move to other communities.

This attitude is exactly the problem. The current situation is new and unprecedented. Not seeing the difference is the type of ignorance that allowed this to happen in the first place.