Imagining Meta wants to expand into another platform isn't a conspiracy theory. For one, Meta could paste ads into more online spaces. They could also replace twitter without having to develop their own platform or pulling a Musk. Both of these would, yes, allow them to be more profitable.

Let me give a hypothetical: Meta makes their own nice, QoL-rich instance that could integrate with Facebook/Instagram. They could also add in analytics and ads and allow that to federate with other instances. They could allow other people to host their own version of this Metadon. If it gets adopted (because it "just works" or otherwise), they could cut support for the instances not running Metadon, taking a large portion of the userbase with them. They would have their own twitter clone (complete with users), they hardly spent time developing it beyond loading Mastodon with their crap, and they would have other people also hosting Metadon (and their ads) without Meta paying a dime.

If Meta does get a sizable userbase then they can absolutely leverage that to force other instances to play their game or defederate.