Only problem: if a user (A) signs up for signal and then uninstalls, the phone number is still tied to the signal app. This means that a signal user (B) now can't get messages to user A. The messages gets delivered to a dead signal account. User A must manually deactivate her account, which she is not even informed about when deleting. Also: if user A has her number trabsferred too somebody else (which is often the case with e g work phones) her account is now owned by a random person.

Whatsapp doesn't operate as a standalone client either.

The parent talked about signing up for Signal requiring a phone number. The following point is tangential from that after the signup is completed using a phone number. Signal Desktop is a standalone client that can be launched and used without having one's phone nearby or available or connected to the net. WhatsApp, however, requires the phone to be around and connected for the desktop app to send the chats through. From that perspective, WhatsApp actually offers a worse user experience than Signal.

I never said it did, but I don't use Facebook properties to begin with.