My girlfriend is Brazilian and she uses WhatsApp a...lot...
I would say the problem with this idea is that your family is often just one group chat. She has maybe 50 threads of different groups in WA, for her moving off WA would be near impossible not because all the groups she communicates with are on there. The other thing to keep in mind is, the "HN Crowd" knows the full implications of having conversations on WA, for just about everyone else they just see a really convenient app that works extremely well. After using it I concluded that it works better than the native iMessage app on your iPhone, which just speaks to how good the app is (in terms of being an app).
The argument to move people to another app is really uphill battle because it's purely ideological/data privacy/etc related. I could easily convince someone to move to Slack from Teams because Teams sucks as an app. But the issue with WhatsApp is that as an app, it's one of the most well build and best performing.
This is the thing. WhatsApp works really well. And so did Facebook groups. I think once communities are established they stay where they are. So anything new is for new usecases. Trying to get people to migrate isn't the answer. It's about new solutions or habits solving specific purposes. For me it's a lot to do with the fact that each chat itself in whatsapp isn't threaded. You can create communities but that's not right for informal stuff and then I can create separate groups but then it gets confusing. So actually you need topic based chat. Ability to create new topics in the same groups. Basically slack or discord for family and friends.
Sorry, but your whole argument is just a basic rehash of all the standard excuses that people give themselves to avoid having to make difficult choices and just sticking with the status quo.
There is nothing "impossible" about leaving Whatsapp. I'm also from Brazil, I also had the "family chat group". My cousins are still my cousins after I left the group. I can still talk with them through in person, on the phone, email, SMS. The fact that some of them don't want to join Matrix is not a deal breaker, just like my refusal to be on WhatsApp should not be a deal breaker for them.
I would say the problem with this idea is that your family is often just one group chat. She has maybe 50 threads of different groups in WA, for her moving off WA would be near impossible not because all the groups she communicates with are on there. The other thing to keep in mind is, the "HN Crowd" knows the full implications of having conversations on WA, for just about everyone else they just see a really convenient app that works extremely well. After using it I concluded that it works better than the native iMessage app on your iPhone, which just speaks to how good the app is (in terms of being an app).
The argument to move people to another app is really uphill battle because it's purely ideological/data privacy/etc related. I could easily convince someone to move to Slack from Teams because Teams sucks as an app. But the issue with WhatsApp is that as an app, it's one of the most well build and best performing.