That's all great, but then there are the times when they don't offer to copy the credentials where they should. Maybe the "correct" URL was too narrowly defined to be useful, or was taken from the setup context and is otherwise wrong for regular usage.... maybe the site changed their authentication process... etc. In the end, all of this tends to defeat the very resistance to the manual entry impulse you describe. If these password manager entry systems worked more flawlessly, your point would carry more weight... but having to defeat the protection your assertion relies upon is commonplace enough in legitimate purposes that it may well be nullified at all times.
In the end, as long as a site is going to use username/password authentication there will always be the need to educate users about what to expect sans the aid of tools.
In the end, as long as a site is going to use username/password authentication there will always be the need to educate users about what to expect sans the aid of tools.