I hear you. I also don't have FB or X or Instagram. They only reason why I still have WhatsApp is my mum. She just doesn't get Signal for whatever reason. She's pretty much the only person I communicate with via that app.
Discussion
Just tell her Whatsapp was rebranded as Signal. UI is 90% the same anyway. 🤣
I have just one friend who always refuse alternative, then remain the issues of groups, like colleagues for sport activities or parent groups for school. But I send SMS and don't care about them of they are not willing to do an effort.