I think you're missing the part where they are in the trying to figure out how to relax that constraint phase (they have not yet) and having trouble in paradise.

They've run into all the issues and nuances elucidated in this thread. They have been receiving pretty intense feedback from people who have stopped using their product because of the concessions made. They've clammed up in response and are losing even more people because they are not clearly articulating the changes to their users (many of whom would be fine with it if communicated transparently and respectfully). They have people who desperately want usernames but also not if it means what Signal is proposing and admit "okay, you heard my request and tried, but hmm let's not do that I don't like this PIN UX and it's not what I wanted when I said I want usernames". And they even saw their product forked the minute it became clear what they were doing: https://getsession.org (blogs start Dec, 2019 which is around the time Signal started messing around with secure value recovery stuff, at least publicly).