No one wants to leave because nobody is leaving.
You're still on WhatsApp and Messenger, they can still contact you that way, so why would they bother changing?
Pick a date, a week, a month, whatever, from now. Tell those you message regularly that you will be deleting all Meta apps on that date. Make it clear that that includes WhatsApp and Messenger. Explain why. I just linked some articles instead of having a long explanation but it still made it clear why.
Let them know they can contact you via Signal/SMS/email whatever you use after that date.
If the informed don't take the leap, the ignorant never will.