This is naive.
First of all, I'm a big fan of Signal and Matrix, but it is hard to overstate how much more polished WhatsApp is in comparison. There's just a lot more friction for grandma. There are also considerably fewer resources behind the alternatives, so things like this [0] happen (tl;dr - it took Signal over seven months to fix a bug in the Android client that would sometimes send messages to the wrong person!!)
Second, the problem doesn't end with grandma. It's the network. There are literally billions of people on WhatsApp and there are places where WhatsApp is the default method of communication for everything. Schools and kindergartens use it for announcements and to communicate with parents. Service providers give out a WhatsApp number to contact support. Friends and family are all on WhatsApp in established groups with a long history that everyone wants to keep. Trying to switch everyone over is futile. Not being on WhatsApp isn't an option. It is that big.
[0]:https://github.com/signalapp/Signal-Android/issues/10247