You miss-represent the fediverse. Users aren't locked in. If someone buys one instance and you don't like it, you move.
Maybe you will but people in their majority won't do that. It's too much operations. Also they might not even be aware of the transfer. Plus if an instance offers good services there might be a technological price to pay for leaving, like leaving instagram. All instances are not equal, specially if there are more interfaces in preparation.