If they don't want to talk securely to you, then they don't want to, why do you want to force them to? They will spill secrets to other people all the time, using Signal or not. They will talk to other people about things you thought was a secret between you. They don't patch their computer. They use trivial passwords to their computer, they reuse passwords on all websites, they don't encrypt their drives or their Micro-SD on their mobile phones. Why should they care about things you want them to care about? They won't.

I use WhatsApp to people who don't care about security, and I can only be contacted by Signal for business stuff.