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?