Folks, I have finally figured it out.
Have a baby.
Since having a baby a week ago, all of a sudden everyone is willing to install a decent messaging app in order to receive pics of the baby.
We explained that we weren't ready for images of our child to end up in the wrong hands via non-private apps. Another thing was telling them that the one single friend who had already got on board with this had already been recieving pics...
It's been a conversation starter for many and I think seeing privacy from the point of view of a newborn has helped our family and friends understand it a bit more easily. Plus they've had to put up with it if they want any photos, so they will see it working firsthand.
So, if you want to have a baby, know that it can be a wonderful opportunity to help loved ones communicate more privately.
It also increases the sum total of love, community and compassion in the world and in your own life but that's a conversation for another community :)
Edit: If anyone has good tips on how to share a little one's journey more privately with those that care about them, please post them in the discussion.
No one cares. Nobody around you understands the security, the need for it, and the requirements. They will pretend, to see your kid. And then immediately and completely stop caring. It works for making people adopt your favourite messenger, yes. But nothing else.
Well, I disagree based on personal experiences. It doesn't work for everyone, but I'm not trying to help everyone through this method
I too, like OP, thought I found the grail when I got my kids. People suddenly accepted using my communication preferences. Only to find years later that they didn't. They didn't care, understand, or respected my wishes. Don't fool yourself: some people do care, but that is 10% tops.