I credit a good part of my success bringing friends and family over to Signal to the fact that it emulates what ordinary people are used to: a centralized service where people's identities are associated with phone numbers. No need to teach them anything new, just download it, punch in your number, and then punch in my number. I think Signal is targeting exactly that and putting more anonymous and decentralized models way on the back burner. Concepts as simple to us as 'instances' are surprisingly difficult to explain to newcomers, and I wouldn't be surprised if accounts not associated with phone numbers pose a discoverability issue.

This all might be sidestepping the question a bit since I haven't dug deep into the issue, but my thinking is that Signal, in its current state, should be seen as a transitional solution until things like SimpleX become more mature and widespread.