Another option is to make migration of everything from one instance easy and let them buy whichever instances they want but let the users go somewhere else. Turn their weaponized capitalism into free money for instance admins until they wisen up and stop throwing money at it.
Or set up the terms and services to give the instances responsibilities that must be honoured even after they get purchased by another entity such that buying them becomes unattractive. Like mandate a certain portion of the topmost parent company’s profits (along with clauses to prevent Hollywood accounting from dodging that, maybe say revenue instead of profit and all related companies instead of just the topmost) must be invested back into the fediverse and that changing the TOS to remove that requires a certain number of users to agree. Set it up so that it is designed to only work if the whole point of the entity is to host a community rather than extract profit from hosted communities.