That's a weird question to pose frankly. US family plans have almost always functioned by discount per phone added to the plan. They average out to being cheaper per person, not more expensive, which is the entire point of the family plans. That's the direction this will move for households, just as it did for each previous unlimited thing (minutes, texting, etc).

To answer it though, the plans will take off and will be extremely popular. This round of competition will be to push up data limitations and speeds, as infrastructure deployment is mostly saturated. Unlimited data will become nearly universal and the speeds will increase significantly.