I imagine if Lemmy/the fediverse get big enough, corps will make accounts. It's a small step from there to making their own instances, then modifying those instances so that they can see out, but you have to be registered locally to see in. I'm not sure how difficult that would be to implement.
But if they behaved like this, other instances would just block them from federation ("seeing out" to other instances) and they'd be isolated in their own little silo. That's the beauty of federated networks - bad actors will be naturally pushed out and isolated.
There’s also the possibility of just paying off owners of popular instances in exchange for allowing advertising to its members and installing trackers.
True. This might be less of a problem if it becomes really easy to transfer your account to another instance, because people would just drift away from the ads, especially if other instances started creating filters to block them.