Is there a reasonable alternative, though? Email addresses? Adding a cryptographic challenge to prevent somebody from generating tons of accounts?
As far as phone numbers go, I'm not a big fan of Signal having them, but I definitely prefer not having to give them out! That change is a huge deal to me, as I can now communicate with people without handing them a phone number. And Signal has provided their client and server source code, along with evidence that their servers store absolutely nothing.
Nowadays, the most likely way your Signal data will get leaked is if somebody screenshots it.