I believe the basic reason Apple delayed implemening the proposed iOS 14 IDFA policy change is that Apple has their own ads business for promotion of app store installs which was using a device identifier in exactly the the same way as other publishers for conversion attribution. However permissioning for this sharing with Apple is controlled under a different "default on" setting under Settings > Privacy > Apple Advertising than the one proposed for IDFA which is case by case permissions request. From what i am given to understand, due to this dichotomy, ad networks threatend Apple with anti-trust lawsuits if they went ahead as a classic argument of using an advantage in one market (device and OS they control) to unfairly shut out competition in another (ads) could be made.
Now Apple will probably disable or remove this dichotomy in some way before they roll out the policy change but they probably will still have other problems. For instance their skadnetwork API [1] which they use for their app installs ads business and recommend to other ad networks supports only app installs conversion on their own app store and not other forms of conversion (eg. buying in e-commerce etc). But if they now restrict other forms of conversion attribution that used to be possible previously, could an argument again be made that they are using their dominance in one field to unfairly kill competition in another.
I think the only way Apple can implement this policy change without wading into an antitrust minefield is to completely give up their own ads business. But i suspect it's quite lucrative, otherwise why wouldn't they already have shut it down?
This is by no means a done deal. Let's get out the pop corn and watch the fun.
[1] https://developer.apple.com/documentation/storekit/skadnetwo...