The most predictable early result is that the most major apps - the true behemoths, which pull users - will all move to their own stores, to avoid dealing with Apple (not just Apple's 15-30%, but all the other rules they'd like to avoid). Facebook/Messenger, Microsoft, Epic, X (for now), Google... the list is pretty long. Smaller (though still major) apps won't be able to pull out of the app store, at least at first, for fear of losing downloads/sales - but Facebook can be quite confident that if you use Messenger, you'll go get if from Facebook's app store if you have to. You won't just stop using it.