Even without looking at the numbers, this idea makes so much sense that it's unbelievable HBO (or Showtime, Starz, etc.) haven't done that already. Cable companies are a middleman, owners of a dumb pipe that offers--on its own--no real value to consumers, and no longer offer a distribution or platform advantage to content publishers.
I can no longer justify paying the inflated bill Time Warner Cable sends me. Not "afford", but "justify". So, just last month I decided to cut out all the Premium channels, simply because they do not offer me the flexibility I need in a paid service. Once these services are available elsewhere as stand-alones, I'll be the first to jump that wagon.
I'm not one to link to my own writing, and usually hate it when others do it, but just days ago I wrote something that ties in nicely, from the POV of the cable company, so apologies for the shameless plug: http://triggered.roundedsquare.com/2012/06/if-only-time-warner-cable-was-more-like-a-restaurant/