yeah, this part itself. How easy it is to get them to do it?
Probably that they don't bother to have another chat app just for a few people.
A older family member has WhatsUp and Signal and gets sometimes confused. I have WhatsUp, signal, telegram, discord, meta messenger and would be .. hesitant to install an IRC client for like 3 people.
And lose because you can't give it to a kid that doesn't have a mobile phone number.
I have shared custody of my daughter and we communicate via xmpp on a tablet they carry over there when they spend a week at their mother's.
Want to find that recipe that you remember that some person shared with you a while back? Now you have to look through four different apps.
And the overhead grows quite fast as you add apps.