But for solutions, can't you just buy a voip number? You just need it for registration and then can dump it. I'm sure you can buy one with cash or zcash if you're really paranoid.
While in the US I don't have to show my gov ID to get a phone number, I don't know anyone who buys a phone with cash except international students. So practically everyone is identifiable anyways. But I'm not sure this is a deal breaker since all I'm leaking is that I have registered a Signal account. AFAIK Signal only has logs of an account existing and last online with 24hr resolution (which avoids many collision deanonymization methods). Even paying with cash is hard as I'm probably caught on camera (but these usually get flushed).
So I'm legitimately curious, why is this a dealbreaker? It doesn't seem like a concern for the vast majority of people, and the problem Signal is solving is secure communication for the masses, not the most secure method possible with unbounded complexity. It's being as secure as possible while being similar in complexity to the average messenger.