If a corporation aims to purchase an instance, all the other instances would let them know they will vote to defederate it as soon as the purchase is finalized. That ought to make them change course.

Or, they just won’t care

Maybe. But then the instance they purchase is sealed off from the rest of the federation and a large portion of the users jump ship.

That's not how meta's going to do it; they'll start their own (with blackjack blah blah) that has better servers, better design, and other stuff real money can buy until theirs is the default. Then when they have the vast majority of users, they'll start throwing their weight around and others will be pressured to comply with the 'standard' they set.