Heck, my microsoft account is tied to an email I can no longer access, but I'm not able to change it, because my generic gmail identity somehow got an xbox account tied to it and that apparently means I can no longer transfer my xbox profile from my old email account to my new one.

My appleId has an @mac.com email address which means it is for some reason immutable - I can not change my address. Jobs only knows where emails from apple to me end up, I really don't do much with my appleid anymore other than re-authorize the appstore every month when I need to update xcode.