You're right that in terms of a fraction of hours of content per month, most of what they show is not first-run original content, but I don't watch TV 24 hours a day, so I don't care. Like I said, if I got that amount of original content from four or five different providers (let alone 10), that would be more television than I have time to watch. The fact that they mostly show movie reruns is immaterial.
So yes, I would like to just be able to buy Game of Thrones, but I'd also, personally, like to be able to just buy Deadwood or (on Showtime) Queer as Folk, and I think if every show had to justify itself to network executives on raw viewership numbers instead of on audience-broadening power, those shows would never get made; their audiences are too niche compared to their production costs. I certainly appreciate the opportunity to see at least some content that's not aimed primarily at 18-35-year-old straight men.
Of course, this is the kind of reasoning that justifies 100-channel cable packages instead of a-la-carte cable channels, so take what I'm saying with a grain of salt. But I think the expectation that a-la-carte show purchasing could really work, economically, is probably not realistic (and this without even getting into the issue that it'd be tough to pay for a $75-million Game of Thrones season at a dollar an episode).
The problem with HBO adopting a pure-streaming service is that it's still more lucrative for them to work with the cable providers.