With these kinds of things, as always, if you have all the shares, you have the secret, and with Nostr the secret is your forever password. And the threshold of signers have to be online to work. So to use this as a password replacement you'd have to put some thought into the setup. Most people will be lazy and have all the signers on the same laptop, but that won't work for when they want to sign in on mobile and their laptop is not online. So cloud then—but it's a little tricky to setup a cloud environment that protects from collusion. So this will take time to get popular.

technically yeah though nostr integrated sites are easier. other sites would need to integrate some kind of nostr native auth, I imagine.