So they could have replaced it with, like, email verification or something, but they instead stuck to the design that lets governments identify all users?

<Insert rampant and unfounded speculation about FBI compromise here>

Pretty sure you still need a phone number for an account, though - the usernames are just for sharing your contact with other people.

Most peoples’ phone numbers are easily linked to their identity. Which means the government knows who’s using Signal.

Usernames are definitely an improvement, but this is a fundamental limitation in Signal’s design.

One use case could be mass protests/uprisings, where you have a lot of people congregated in a small area. An increasingly popular strategy among governments these days is to just shut down the entire internet in an agitated region. Bluetooth could keep information flowing between people with only mutual contacts, as they move in and out of range.

Consider Briar.

Uses Tor. Works directly over Bluetooth/WiFi if the internet is censored or shut down. Decentralized, no accounts. No phone number required.

Of all the options available, I feel like this one is the best suited to current threats (oppressive governments with all-encompassing surveillance, and the willingness to destroy critical institutions and infrastructure).

The app is super barebones right now - feels like SMS - but it works. Main downside is that both participants have to be online at the same time (maybe group chats can work around this?), since there’s no servers.