> If you're so motivated, can even build something for them in open source
No, as far as I know they don't allow alternative clients (at least on their servers). Of course that is their right but IMHO still a dick move (again nothing to do with budget) and we are stuck with the official client. Open source is nice but with this limitation it has lost the biggest advantage.
While you literally can't use WhatsApp without your phone nearby.
Edit:// as for functionality no current IM can beat telegram except WeChat obviously
Tying accounts to phone numbers is a design decision that is just plain wrong for anything that does not only run on a phone or anything that is supposed to identify a person rather than a phone number.
If that is the case, Signal already supports that feature.
Also, I've had no issues with the Signal app on android nor iOS. Neither has any of my friends.
It's also got many compatible Matrix clients (Web/iOS/macOS/Linux/terminal) which don't have to dance around the rules like the unofficial Signal clients.
It also syncs message history across clients (all e2e encrypted).
[1]: https://element.io/
WhatsApp gave us the chance to convert a lot of people to a facebook-free messenger because of their change of terms and we did convert a lot of people.
But meanwhile I have to agree with most people I know: the Signal app is just shitty compared to WhatsApp (often crashes on android, you can't set a custom notify sound, messages arriving late, ...) and now it also lags behind this new feature.