It isn’t productive to establish defense against an arbitrary future that turns on you. Spend those brain cycles focusing on your user and building a great product. Choosing Sign in with Apple is great for Apple users.
It absolutely can be productive. So many people have done as you said and had the rug ripped out from under them by $ARBITRARY_PLATFORM_DECISION (cough cough YouTube) that if they had thought the risk they were taking on by going with one platform they might still be in business.
Maybe so. But how is this different than saying, "I have nothing to hide, so I'm not worried about having my car searched?" It's a somewhat pragmatic point of view but I wonder if it is a good idea in the limit.
So what are you saying then? We shouldn't agree to a system that allows our car to be searched because under some limit it might be bad for us? What are we comparing this to in the Apple case? What is the equivalent of the "car search"?
Against an arbitrary future? No. Against a likely future? Yes.
It is not the hallmark of intelligence to come up with every possible thing that could go wrong and build defenses against it just in case someone somewhere does something counter to their wellbeing.
If you use any third-party login system, think about how you can migrate users between accounts, or validate that they are who they say they are, that's perfectly normal. What if Twitter OAuth goes down? What if your Google dev account is suspended by some automated system for some reason? You should definitely have some kind of plan for that.
Spinning off contingencies in case Apple changes its developer policies out from under you and suddenly no one on iOS can log in? Too specific and arbitrary.
