Since more people are trying to prove you wrong, I’d argue it can’t be too late for Mastodon to fix that username scheme used in the frontend app. And agreed that double @ is stupid.
Mastodon supports URI's in the UI, and resolves @user@host names to profile URI's when you post as it is, so the use of webfinger is almost entirely a presentation/convenience issue anyway. I also dislike the double "@", but I get the thinking that starting "@" has to some extent become a signifier that what follows is a social handle.
