How are those numbers figured? If you have a family of four living in a house (alternately, college students or professionals sharing an apartment), I'd assume you're counting that as 4 wireless broadband subscriptions? Or if you're talking about adults working jobs with business numbers, maybe 6? Regardless, it would be a lot more meaningful to look at whether a given person has access to a wired connection at home, instead of an apples and oranges count of subscription numbers.
If I had to guess about data consumed, that single wired connection probably handles more data than four mobile plans put together, thanks to mobile data caps and Netflix.
As far as broadband having "plateaued," it's not too surprising that something with a theoretical cap of 100% penetration would eventually stop growing. There are only so many houses to connect.
I don't have any particular problem with mobile broadband, my LTE connection on T-Mobile is actually faster than my cable. But there are a lot of people who use more than a couple of gigs a month, so cable isn't going away any time soon. And in the meantime, I'm not thrilled with Comcast having free reign to do whatever they want just because "the market will solve everything." It won't.
EDIT: At 20 Mb/s, it'd take me about half an hour to burn through my entire month's 5GB data allowance