It's not "really easy", though.
I have WhatsApp installed on my iPhone. I talk to one friend in Canada with it, nobody else was interested in using it, and even my Canadian friend and I have moved to Facebook Messenger to keep in touch.
It's not just persuading your family to install it. It's persuading my parents and my wife's parents to use it, who barely understand texting as it is. It's persuading them to remember to text me (and only me) using WhatsApp and everyone else using Messages. It's explaining to them why WhatsApp is a "better" solution, and if it's so much better, why does her church group use the other thing, which works just as well?
It's not just getting your friends and family to use WhatsApp. It's getting your friends and family's friends and family to use it. And that's a tough hill to climb when Messages is right there, default, and works great out of the box. I really don't understand why this is so confusing.
GAF is in it's bubble again. I guarantee you most users don't see or understand the difference between WhatsApp, Messages, Facebook Messenger, or SMS. They use what's right in front of them.
That's extremely reductive.