Steam
might go for it, but I think Epic would be the big one, mostly because I don't believe Valve has any mobile games that would entice users to join their app store. Well, I guess being able to buy Steam games without going through the App Store cut would be a factor, but then it would just be a standalone storefront app for purchasing PC games, not really a shady app store with questionable binaries full of user tracking.
> My main point is that they would just immediately make you sideload to use Facebook
What about my points about the regulators? Meta does not exist within a vacuum. They are under as much scrutiny, probably more, than Apple. Sideloaded binaries will end up combed through by regulators, government watchdogs, security firms, Brian Krebs, etc., to check for Meta malfeasance. Plus, the point about forced migrations being potentially monopolistic behavior.
And again, they can try to make users sideload, but a not-insignificant minority would not comply. Forcing them to jump through new hoops while presenting no new benefits would not be a good user experience. Plenty would take to social media to complain. Many would read up about Meta's dastardly data policies. It would be a PR nightmare. The mea culpas and reversals would come quicker than you can say "Senate hearing."
This is also all overlooking the fact that Meta as a company is in a pivotal business situation right now- DAU was falling a few quarters ago, they just spent a ton of money on the metaverse with no results other than avatar legs to show for it, and so on. Trying to do something as disruptive as forcing their customers to get their apps from a new source could be highly detrimental to them.