they did that already. you can sign up with just an account name now, i ditched my phone number from my account after it rolled out around a month ago.
Still the fact that you can use it without disclosing your phone number to anyone except signal is indeed useful and a improvement from before.
https://support.signal.org/hc/en-us/articles/6712070553754-P...
Yes, all more difficult than using phone numbers and outsourcing the proof-of-humanity problem to telcos all over the world, but in my opinion it would be worth it.