I think this mostly affects small groups. If the group is just 5 people, then one individual can prevent it.
For bigger groups (eg. the parents groups of my kids school) nobody cares if one or two people don't want to move. Especially since there most likely were a few people who already opposed WhatsApp in the first place.
So before there were a few people who weren't part of the group because of WhatsApp, and now there might be a few different people who aren't part of the group because of Signal.
You just need to convince a majority that your choice is better, and the terms of service change along with the constant bad press of Facebook was enough to tip the scales for many many groups.
For the big groups (30+) I didn't even try. I heard nobody talk about. I feel that these groups will treat a message about this as pure spam. These are not in any way tech interested people btw, just parents from the kid's school.
It probably makes sense to find one or two allies in the group (by talking offline) before proposing the change. I think that's how they did it for my older kids WhatsApp group.
(I'm not sure because I was one of the obnoxious parents who refused to install WhatsApp in the first place. I was a bit surprised when the group suddenly decided to move to Signal)
For bigger groups (eg. the parents groups of my kids school) nobody cares if one or two people don't want to move. Especially since there most likely were a few people who already opposed WhatsApp in the first place.
So before there were a few people who weren't part of the group because of WhatsApp, and now there might be a few different people who aren't part of the group because of Signal.
You just need to convince a majority that your choice is better, and the terms of service change along with the constant bad press of Facebook was enough to tip the scales for many many groups.