For many use-cases they are indeed proper alternatives, but Jitsi has a much lower limit when it comes to numbers of participants. So if someone wants to organize a group call (the typical use-case) you come a with a lot more scenarios into trouble using Jitsi. And it hurts me to write this, because I hate how much growth Zoom had in the last year while being dishonest about critical encryption features. I would love to promoted Jisti and have tried so in the past, but I can't recommend something when it doesn't fit the use-case.

Signal, on the other hand, probably comes close to the WhatsApp features (I use neither one) and while I encourage everyone to to switch, I am missing the federation aspect. IMO, communication should be federated by law (which would also solve the network effect problem). Imagine a world where you could not call someone who has a phone number from a different provider? The current state of instant messaging is exactly this.

XMPP solved these problems decades ago and just because the standard didn't catch up with the speed of the mobile revolution, we don't have to reinvent everything from scratch. Properly implemented modern clients work very well (including reliability and battery consumption), the big issue though is that many traditional clients don't support all features and all companies in the business try, to build walled gardens as those tend to driver stock prices.