Could a "login with lemmy" replace all the login with Google/Facebook we see everywhere?
I love the convenience of not having to create a password everywhere I need to be authenticated. It would be interesting to be able to use lemmy instead of feeding more information to these big corporations.
No, because Lemmy doesn't support Oauth2 yet. And even if it would support it, at most it could be a "login with lemmy.ml" or similar instance specific button as the protocol requires a specific endpoint.
Edit: see other comment in this thread. With the OIDC discovery extension to OAuth2 it might be possible, but I haven't seen that feature being used this way in the wild yet.
Yes, but that is not how Oauth2/OIDC works (the old OpenID did, but it has been largely abandoned).
One of the reason this approach was abandoned is that these external login automations are very easily abused for spam if you allow arbitrary instances as the auth endpoint.
The old OpenID didn't see much uptake (because of the spam issue) and the alternative Oauth2 that was AFAIK mostly pushed by Google is clearly designed for the purpose of large centralized providers. So I don't think there is a direct causality, but yes it is related.
Never the less Oauth2/OIDC works quite well and is clearly better that most of the alternatives still commonly in use.