Tying to phone numbers makes identities expensive.

What's the threat model that requires multiple identities?

From privacy point the standard is using separate identity for every action, if possible. - That's also a major bummer for Briar.

That's not a threat model, and not generally possible (or even desirable) with something like a messaging app. So again, what is the threat that you wish to address that you believe is only solvable with multiple identities?

This is an important question, because cryptographic repudiation and secured metadata prevent most of the dangers that I can think of, but I might be missing some.