If Meta pulled from the official app store, their install rate would go to 0 overnight. And a third-party developer would quickly swoop in and take over the #1 search result for the word "Facebook" (plenty of such apps already exist), so oblivious users would still end up ahead.
Sure they could offer a sideloadable app on their website somewhere, but nobody will find it or use it. If your grandma searches for facebook and the top result says "Friendly for Facebook" instead of just "Facebook", do you really think she'll notice the difference let alone go on a wild goose chase of googling for alternative app stores and clicking through scary warnings?
I mean it didn't stop a lot of people from clicking through "scary warnings" to install Facebook's VPN app that hoovered up their data. I think you are vastly underestimating what people will go through for a minor benefit to themselves (in the case of the VPN it was low dollar amount gift cards).
FB won't remove their app from the app store but instead will add a new feature or offer a perk that is only in their side-loaded version and it will drive users in droves to install it.
Sure they could offer a sideloadable app on their website somewhere, but nobody will find it or use it. If your grandma searches for facebook and the top result says "Friendly for Facebook" instead of just "Facebook", do you really think she'll notice the difference let alone go on a wild goose chase of googling for alternative app stores and clicking through scary warnings?