I agree that user@domain will be misunderstood as an email address, which is a usability issue.
I don't agree that @user@domain is up for grabs. As others said, Mastodon uses that.
I agree that it is largely a branding/usability issue.
I don't agree that a URI scheme is a usability disaster. I think the @user@domain is a usability disaster. I don't think a slightly longer string hurts usability as much as a mastodon-looking string that soapbox says is formatted correctly but oops it cannot connect to the server.
I don't think I'll have any more points to make to any reply. I'm barely interested enough even to make these points.