I miss when OpenID was a thing and I could run my own iDP, but that didn't catch on because everyone wanted to be the iDP and not an RP. These days it's a cabal of a few big companies.

yes, this could be designed much much better.

More convenient. No passwords, and better security. Everything is centralized (disadvantage too)

Also,

"why people would use anonymous email for services like this? I always use oauth"

as for password, I use password manager.

Problem with oauth that I can't make e-mail unique and I can't know where spam is coming from.

ex: user+dropbox@mydomain.com will be unique for that domain.