They've been working on it for years. Their solution is that they have to take client-side ownership of your contacts list, keep it associated with your "account" and sync it across your devices so that when you correspond with someone by username, it becomes available to you everywhere. They have to be your contact book. I can find nothing on how they plan to verify usernames, perhaps in the traditional style with email.
So yeah, absolutely not some trivial change that they just don't want to do because fuck the few people that don't have a phone number (or don't want to use it). They're working toward supporting usernames and at every turn keep getting reamed by HN because, in their effort to solve a problem that only exists on HN, they have to deploy a solution that means you have to trust them in a teeny tiny way you didn't previously IF you set a weak pin on your account. It's mind boggling. It must be so disheartening to see that type of response.
But, that's my point. Signal can't add short names without changing the fundamental trust model which appealed to everybody initially. No amount of hiding a password as a pin, will change that. I really hope they don't kill their product along the way...
(Also man WTF they're running Raft on SGX enclaves just so they can rate limit attempts to brute force users' weak pins. While super cool, technically, what an incredible waste of resources just to try and make weak passwords okay. Probably the most backwards thing I've seen a security company attempt like ever. Just tell your users if they want a username they need a strong password. Or just generate the entropy for them and only allow the username option to people who also want to take custody of their new 32-bytes of entropy and have a signal-managed synced contact book.)