The issue is WhatsApp from a few years ago versus WhatsApp today - meta/favebook is probably the worst possible company to have bought it, their record on security, privacy, and features is horrible.
The app today is full of enshitification (meta ai being shoved down my throat by the communication monopoly) and nobody can ever fully trust their security or privacy because its not open source.
Signal sadly doesn’t yet have the ubiquity of whatsapp - but for everyone that has it (now I’m finding even non tech-savvy family are switching over) use signal, and where you have to, use WhatsApp.
as a big proponent of FOSS I see where you’re coming from - but the reality will always be that apps which have a significant learning curve to even install are obviously hugely off-putting to the majority of users. While the rest of us might be comfortable cloning a repository and building from a tar file, expecting the average person who wants to talk with friends and family to jump through those kind of hoops is exactly what has held back wider adoption of better standards.
Things like flatpacks and snaps have gone a long way to making this less daunting, but when matrix isn’t a ‘self-hosted decentralised chat’, it’s a *‘version of whatsapp that isn’t always online, and i don’t know where to download it and have to learn what the terminal is to even get it on my laptop’ * - we can’t be surprised people stick with the less secure, private, easy options. That’s why I’m a big advocate of signal - it’s not perfect and part of me wishes it was matrix or threema or one of the other standards, but getting people comfortable with the idea of free and open source software, while making it as simple for them to install on their phone or computer as anything meta makes is a really good first step - in the meantime, it’s up to us in the wider community to make the other solutions more intuitive, simple, secure, and trust that if a good enough job is done of that - they will come.