They are working to remove the cell phone requirement. https://signal.org/blog/signal-pins/

"PINs will also help facilitate new features like addressing that isn’t based exclusively on phone numbers, since the system address book will no longer be a viable way to maintain your network of contacts."

I suspect you are massively outnumbered by people who do use it because it integrates with their existing phone book, and is a drop-in replacement for the default SMS app. Without those things it'd be just another niche app for weird nerds.

If it didn't have this feature of being a drop-in replacement for SMS, then I would be using Matrix to communicate with my two privacy-minded friends, and plain-text SMS to talk to everyone else in my contact list. With this feature, I've been able to convince about half of my Android-using contacts into switching (iOS is a harder sell).

I believe the point was that phone numbers are already used as unique IDs people know intuitively. Essentially make it quick to onboard new users

Thankfully they are moving away from this with the option to use a PIN instead pf phone #. Not sure when that rolls out, but I'm v. excited for it

I think they justify sacrificing privacy like this for the sake of growth (which of my contacts use Signal? etc).

Users a phone number. Doesn't have to be your main number. Setup with a burner sim and throw it away.

it's their negotiation of the bare minimum invasive activity they've decided they must do to give us some threshold of network effects in an otherwise anonymous service

The whole point of Signal as far as I have ever been concerned was to encrypt one's SMS communications. How would you do that without a phone number?