You don't have to offer Sign in with Apple if you have your own login system. Ref: https://developer.apple.com/app-store/review/guidelines/#sig...
This is only applicable if the Apps provide this as an option right?
https://developer.apple.com/app-store/review/guidelines/#sig...
I wouldn’t treat it as a privacy feature, I would treat it as a convenience feature. When I needed some extra mask straps, I installed Etsy, didn’t previously have an account. So I used Sign In with Apple to register, picked out some mask straps, and Apple Pay to pay. The whole process only took as long as it took for me to make a selection and two biometric authentications, but some of my information had to be exchanged in order to make that transaction possible.
Also I can login to UPS and Etsy without a password.
All of the Apps get the same ID and that's how your "persona" graph is constructed.
Sidenote: One of the bets people are making with this change is that now free apps will start forcing you to signup (via email/mobile number) which will be used to construct this "persona" graph again because who has time to create new emails for these purposes.