This is what I’m worried about. As the fediverse grows and gains popularity it will undoubtedly become worth targeting. It’s not hard to imagine it becoming a lucrative target for things like astroturfing, vote brigading etc bots. For centralized sites it’s not hard to come up with some solutions to at least minimize the problem. But when everyone can just spin up a Lemmy, Kbin, etc instance it becomes a much, much harder problem to tackle because instances can also be ran by bot farms themselves, where they have complete control over the backend and frontend as well. That’s a pretty scary scenario which I’m not sure can be “fixed”. Maybe something can be done on the ActivityPub side, I don’t know.
What if you have 100s or 1000s of such instances? At some point you defeat the entire purpose of the federation.