There's a lot of "this or that" in the technology space. I use OAuth and classic username/passwords for authentication. For really important services, if possible I use multiple methods. Some websites are smart enough to allow you to connect one login type to another, some don't.

The reason I'm saying this is because, if public key login is supported, and key management is hard, there's no reason why an alternative username/password method couldn't be also supported alongside. That way you lose one method of connection, you can fallback to another one.

Would you need this for all services you use? Probably not. For example using a public key login on something like hacker news, and being required to create a new account when I lose that? No big deal. Losing my public key when trying to login to my back account, well let me fallback to a "forgotten public key flow" that uses my email address to set up a new key.