You're forgetting that apple 'vets' apps and can reject them for arbitary reasons. They're in a position to say 'you must use our Ad-ID, and if you don't and w

You're forgetting that apple 'vets' apps and can reject them for arbitary reasons. They're in a position to say 'you must use our Ad-ID, and if you don't and we detect you tracking users by other means, then your app will be rejected'. And as it happens, that's what they do, when they introduced IDFA they started rejecting apps that use other tracking heuristics, including the old UDID system.