No one knows if corporations have a way to mess this up yet, which has in turn fueled the push against META joining the space.
But by design it is safeguarded, it is all about horizontal growth, instances aren’t thought to grow indefinitely so it is encouraged that people make their own instances to spread the load. As long as we have this option a corporation can’t take over since as you said everyone can just defederate, or if they are part of an instance that has been corrupted by corporate instance you can just move over to other instance. This is the key difference with a centralized service, in reddit yes you have different subs with different rules and moderators , but at the end of the day you are all under the whims of reddit and have no sub to escape to once reddit fucks ip. Here if the equivalent of reddit where to fuck up, start banning people or defederating with everyone once it grows big enough, you can just move to other instance inmediatly and aboid the problem.
That is why I’m not really that worried of corporations making instances regardless of how big they get because as long as the protocol remains unchanged we can all just flee with no issue.