Hmm. Problem with this approach, as with Sign in by FB/Twitter, is: what to do when the provider bans you? People have complained since years that Google takes down their entire account including mail for stuff like "violating copyrights" on Youtube while being compliant to fair use (and then doesn't even offer you to get your data!), and Facebook and Twitter are home to troll squads abusing the "report" feature, which triggers automated anti-spam/troll systems before a human actually can do a review.
What's worse: Google, Amazon (yeah they ALSO act as a SSO provider!), Facebook and Twitter offer their users no way of appeal or even contact with a human in case of a ban, and not even when you get a lawyer to write a nasty letter to them. In contrast, regulated telcos (in Germany) have very strict requirements on when they're allowed to kick off customers. Time to regulate the giants in the same way, given their importance to not just communication across society, but also due to their growing SSO power.