but now i'm already imagining fb app will be from alt-store with all the tracking reactivated and yes i don't use fb/have the app but try to explain in to elders/no tech people
If Facebook truly does use this as the opportunity to ruin their UX, then maybe your parents switch to Signal or iMessage.
This is what's scary, because it means that Facebook has what's effectively a massive captive audience with its messengers.
But, they don't. Realistically there's no reason for them to exist anywhere besides the App/Play Store, as long as those platforms play fair. This is a real strawman argument though, considering it's fully possible in the status quo. The problem isn't Apple or the government, it's that Facebook already has a disproportionate amount of control over your life. Apple cannot save you from that with a software update.