1. Consumers are already sick and tired of having to do account management already and having to deal with Meta or Google forcing them to do more of that will lead to backlash.
2. I'm unconvinced that Meta and Google have the product capability wherewithal to force a great migration, judging by their attempts at new products in recent years, let alone what basically amounts to a new platform. The Facebook Home launcher went nowhere and I'm pretty sure Android Instant Apps didn't either. They don't really fare very well outside of their home turf, and iOS is definitely not theirs.
3. Regulators, particularly in the EU, would not sit idly by while Big Tech lures customers to shady third party app stores; if anything, these stores would present opportunities for greater scrutiny and more crackdowns.
4. I'm not even sure if Meta can get away with pulling a crucial app such as WhatsApp away from the main App Store. That seems like monopolistic behavior, and disruptive to the lives of tens of millions of people who use it for basic communications. The EU would probably mandate them host it on their third party app store AND on the official App Store.
5. I do have a carve out exception for games, because publishers regularly engage in extractive abusive behavior and gamers just take it or pirate:
https://news.ycombinator.com/item?id=30204012
I do think the possibility of a Bytedance app store/sideloaded .ipa is a wildcard, as TikTok seems to be the current killer app on mobile.