I know many users will be "Oh shut up, you just don't want to accept it", but there's a bit of a valid argument here. In a ways, it's kind of discriminatory where if you wanted to make things easier for your user by adding a single sign on kind of solution like FB/Twitter/Google, you must add Apple, or you're stuck with regular ol' username/password. Otherwise you're booted from the service. "Other services are optional, but we're mandatory". It's kind of like saying "If you want to use your Visa or MasterCard here, you're required to have a store credit card too, even though you might not use it. Otherwise, just accept cash."
It's stripping away the liberty of the developers to add services without adding a native service so that they're always there. I personally like SIWA and use it, but it kinda sucks that it's such a heavy requirement for a sign in feature. I know I'll get flack from users (heck, Apple users are already becoming a little skeptical of 3rd party developers), but to quote Marilyn Manson "something beautiful, or something free".