I agree that trying to force people to move like that is a way to lose friends.
The way I did it was by gradually spreading the word a few years ago, until probably half of my friends were on Signal after a few weeks (I have a fairly technically orientated set of friends so that wasn't too hard)
For the rest, it was about waiting for a good time. The recent media coverage about WhatsApp T&Cs (regardless of how accurate that coverage was) ended up being a good way to have a conversation with people about moving and most others either moved or already had Signal by then.
For the last handful, the conversation has been "Have you tried Signal yet? [ ... usual blurb...] Anyway, I'm on the verge of deleting WhatsApp now, so you can get me in future either with Signal or SMS.
I'm now down to 2 contact that I use WhatApp for and I expect to have deleted my account in a few weeks.
At no point was any pressure or anything like that needed, and well timed gentle nudging works and I believe is more effective than lawsuits.