I've had some luck getting some of my people to switch over by asking them to switch as a favor to me specifically (if they want to of course... And they can always just not use it if they end up not liking it).
Asking them in a more direct way like this seems to work, even if there's some reluctance at first. I tell them that it helps me out because I can also send stuff to them on my computer, which is true. Then there's the fact that I just really like all of the extra features it has (it's just a really good chat app in general), emoji replies, reply quotes, and read receipts to name a few.
If they need any other reasons, the last things I tend to get into are the privacy/security aspects.
Boom, all of a sudden they're using something better, and even though they're probably still using those other (shit) apps, at least I don't have to. You have to start somewhere, right?
I like it that it really pulls in the focus to the most important aspect, great work!