When you use Google or FB or others to sign in, you just get some data, that you can trust.

Internally, on my website, i may have an account, that i then link to this Google or FB account.

1) If Google shuts down an account, authorization might still work for the purpose of logging in somewhere else. Your email might not work anymore, like any other services within Google. But authorization does. That it does not, is an implementation detail.

2) Since internally i have created an account, that is only linked to your Google account, i can always allow you to also login via any other method. Maybe with your facebook account.

3) I think, things will become better. You use your devices to authorize yourself. And i then trust your device. Then there is no 3rd party involved anymore.