As stated in previous replies, Signal hasn’t been compromised. So, an argument could be that you can talk about more sensitive discussions, i.e NSFW, or controversial topics, without the information getting sold to third parties.
Another way, could be to get them to try it. Once they have downloaded, and created a account, they will use it for communication with you. And because more people will have it, they might communicate with each other on Signal instead.
Here is what I would say:
“Could you at least try Signal? You could use it for a week, communicating with me, and if you run into any problems, you don’t have to use it.
Say this to your friends and family. The more you get into Signal, the easier it will be to get others to use it.