Tangentially related, I have to take issue with this:
> addressing with user-owned identifiers like phone numbers
Phone numbers are owned by telecoms, not users. Sometimes they're transferable between telecoms, but not always. I, in particular travel internationally often and do not maintain phone service in the same country continuously. I've had to change phone numbers with Signal and Whatsapp a couple times now and have not found it to be a particularly friendly experience.
I got a free Google Voice number and might use that in the future, but I had to tie that to another US-based phone number. What will happen if someone else starts using that number, especially if they also connect a Google Voice account to it?
I don't know that I have a better design in mind, but using a phone number as an identity has some nasty edge cases.