That would be painful to use in the browser, as you can’t authenticate with SSH on a browser, and TLS certs are a pain to setup and maintain, compared to OAuth (the only time I had to use them was with StartSSL before it died… and it was not a pleasant experience).
What’s unsafe about OAuth? Personally I don’t know of any critical technical weakness of the protocol, but I’d love to be proven wrong here.
Usernames are actually pretty useful: you can easily see the other crates created by that person, and you can have a name to ping if there are any problems (like what I do with Crater).
I don’t think supporting arbitrary identity providers is going to happen, as it would make the spam/squatting situation way worse than it currently is (at the moment we rely on the third-party providers to provide another layer of protection against abusers).