I mean, I try every now and then to get someone to write me on Signal, because they won't find me on Whatsapp, but even once close friends don't seem to find it necessary and continue to use Whatsapp. Guess I am not important enough in their life. Others are a little misinformed, they think using Signal is just a German thing, but are willing to try. Others have their entrenched messenger being Whatsapp and they will take that to their grave with them, before they try anything else for a person they don't know well yet.
With some people it worked though and we are using Signal for some time now. Maybe it is too much to expect a 100% success rate for switching.