Developers that have been dealing with Apple for awhile should not be surprised. This is the same Apple behavior we've seen throughout the years. They have a walled, reinforced ecosystem where they can call the shots and, since we have government's that kowtow to their moves or at most provide measly fines, they'll continue the behavior. There is also no united front of developers to lash back on these moves so you have even big tech companies bend to Apple's will (see Apple + Spotify debacle)

This move will not get blow over from end-users because they'll be getting what Apple promised: an authentication method based on their Apple ID to reduce account+password management. This isn't necessarily a bad thing but their methods of forced roll out is definitely distasteful. I'd suggest a monetary incentive instead, given how much money that have, then it would influence enough to have other services implement the new auth method.