"A lot of people" is a bit different to "most people". SMS is free in the same sense that Whatsapp is free and you need a phone contract to use either when you're not in a wifi network. In fact there was a period where adding 3G to your phone contract was noticeably more expensive.

Regardless, my point is that there is a logical path that some people will take to move to more private options. I'm doing exactly what I described in my comment, a bunch of my friends are doing it and I don't think we're that exceptional.