If you get to use it just your personal computer (any modern OS) without a smartphone, than you would also get to choose your own identifier. For Facebook (WhatsApp) this is a non-starter, because your phone number is a way better identifier to link your profiles and sell your eyeballs to advertisers, so it will stay a requirement.

On a PC you could even have multiple accounts on the same device, which is not good value for advertisers either (one device per person makes it easier to correlate various data sources and track you).

No idea why Signal doesn't enable us to use their tool without a smartphone. It makes it look a bit dodgy from a software freedom perspective.