I think you are right that it will be an issue in the long term if things stay the same. Somewhere I hope this forces the other players in the field to react, to avoid Apple getting too much power.

I don’t see anyone else right now that can force app devs to support this scheme (for instance I saw countless signup pages that reject gmail’s + syntax, companies clearly don’t want to be filtered), nor that would be trusted for now by users to not abuse the situation.

Perhaps Microsoft could ? barely, as they for instance were sticking ads in their browser.

So, in a way I see Apple doing what only them can do, and hope it creates enough precedence for other minor identity managers to step in and being a more sane solution for the long run.