It is true it is relative, and a "power user" so to speak will always use more. The average in Q1 2015
was 2.5GB/month but as we all know, streaming will cause that value to sky rocket. In that same link, Cisco estimates it'll be 11GB/month in 2019. My plan is 15GB/month (with rollover) for 6 people. However, as I'm sure everyone is aware, these values are somewhat bogus because these cell phone data users mostly also use wired/wifi (non-phone network data plans). If they have no wifi, what is the usage? My contention would be that those people who can only afford one service (not two), as my original post suggests, would find a way to reduce data usage over the phone network to stay within the cap. They would go to cafes, or hang out near free service hotels, or Denny's, crap like that. Moreover, regarding speed, they will choose based on price, not necessarily service. And, I have found that at least in our area, phone data service is sufficient for all normal uses (e.g., games, streaming). However, I don't have an HD device connected to phone data service, so I would defer to someone with more experience in that. However, I will also point out that people with less money to spend are far more forgiving to bad service. So, comments like "they won't stand for buffering issues" are false, I think they will suffer through a lot if it means they pay less, because they really have no choice.
(sorry for the long-winded reply)