> one reason is that basic auth lost out early on to site-supplied login forms, so people got used to entering usernames and passwords into the page content anyway, instead of the browser UI

To be fair, basic auth is not particularly user-friendly. If you want to add anything else to the login form, such as a "Remember Me" checkbox or a captcha, you can't put that in the browser chrome, you need to add an additional step in the login flow. If you want an "Did you forget your password? Click here to reset it." error message, the user has to cancel out of the auth dialog in frustration before they can see it (or you have to redirect them to an error page after failed auth, with another link/button to Try Logging In Again.

And even if you solve those problems, the user still needs to enter their username/password when creating their account, and I have never seen Basic Auth used for this scenario.