This isn't really any different than the possibility that someone might forget their password.
Topic on User talk:Tgr (WMF)/external login
The difference is that a user forgetting their password is purely within the user's control, while a third-party going away is something out of their control.
We could ostensibly also use external providers as proof of authenticity but not an identity source - ie. when you click the Google button on signup and Google verifies you have an account with them, we clear the captcha and disable throttling (and throttle on the Google ID instead), but you still need to go through the normal login process. I don't think there is much point to it but technologically it would not be hard.
We can use the external provider for authentication, but still have MediaWiki accounts. Also, if the third party goes away (or the user's account is deleted, or if the user cannot access the external account), the user can use Special:PasswordReset to regain access to the account.