He's written about the why. It's a tradeoff to make it easier for people to switch.

https://signal.org/blog/contact-discovery/

(not just individual people obsessed with privacy, the large numbers of people who can see the advantage of privacy but may not invest a lot of effort to achieve it)

XMPP could utilize your existing email address as XMPP address - that's a solution as well.

To identify a particular user so you can send them messages? IIRC, Signal has never claimed to be anonymous.

True anonymity in a messaging service is hard to achieve, and not even remotely user friendly.

Threema generates random IDs. Users then show each other QR codes and scan them.

If the ID is not unique for each message, then they are still tied to a specific person. At best, the identity is obfuscated.

As a practical example, would you say that bitcoin wallets are anonymous, given that they use random IDs?

Yes, it is obfuscated, the pairing of real person and random id would be much more work, than pairing phone number with real person, and you can throw away and generate as many as you want, whenever you want - unlike your phone number.

Still beats using your phone number for chats, or your SSN in place of bitcoin wallet.

Honestly, I'd say it's much worse than using your phone number or SSN. Sure, an SSN makes it easier to tie an identity to a person, but it doesn't instill a false sense of anonymity in the user.

Again, for the practical example, people bought and sold bitcoins with the broad assumption that it was anonymous. It's a widely understood falsehood, yet new people almost universally believed it was anonymous and safe from law enforcement/the tax man.

Pretending that a random yet static ID adds any level of anonymity is simply dangerous. At best, it keeps honest people honest; like locking your door but not the bay window next to it.