It's too much work to keep track of what phones your friends have. I have 200+ people on my contact list, and I think maybe only 4-5 of them don't use smartphones. It may take off if Apple makes their messenger available for all systems, and people actually download it.

You also need to remember, that a large amount of texters are students. Many of my friends send over 4000 texts a month, while I'm running slightly under 2000. On a student plan, unlimited texting in Canada is just bundled into the voice+data package. I haven't been to the states for a while, so I'm not quite sure how they're doing it.

Even if texting does die, it'll die slowly. It's just too much of a hassle to be a hardcore first-adopter for these changes, even if it is cheaper.