This is exactly the risk faced by instances that are big enough to draw the attention of surveillance capitalists.
They are a weakness in the system, because they are such a ripe target for acquisition and abuse. That's always what happens when a player accumulates power. And that's not a technical issue - it's a policy problem.
For example, if there was an agreed policy that no instance should be bigger than 1000 members (or it gets defederated) that's wouldn't be an issue.