Getting that to work is tricky, but it'd be awesome.
I can imagine a system where users prove possession to OWS of their phone numbers via SMS — as they currently do — and OWS issues them certificates using some sort of blind signature scheme; they can then use those certificates to prove to any server they talk to that they are someone with an identity, and the server can use a subsidiary certificate to demonstrate to other servers that it's acting on behalf of someone with an identity (but not whose identity), and the recipient's server can rate-limit based on that identity, and potentially even record information to aid in manually tracking someone down — without revealing the identity in normal use.
I could be wrong, and I've definitely not proven that it can work. But I think it can be made to.
I think people who are for anonymous use of Signal don't understand that the bulk of Signals users don't want anyone who is anonymous to contact them. If I don't know who you are or I can not track you down, then you can't contact me.