I've read somewhere during the first season of GOT that selling the show to foreign markets only earned HBO app. half of the first season's budget. The first season DVDs/Blu Rays were enormous hit too.Sounds to me like the money lost to downloading isn't enough to make a switch to a more direct subscription model financially viable. It would be naive to think HBO hasn't run the numbers on that, too.
Game of Thrones is the most pirated/downloaded show on the internet, but its ratings keep going up and up. The season two finale was its biggest yet. It makes a lot of money overseas and from dvd sales too. The trend seems to be that cable show ratings keep going up while network show ratings have gone way down.
HBO has zero reason to change anything right now. They are fine with the subscription model.
I would pay a reasonable sum if HBO would offer downloads of their shows, but since it's probably never going to happen, it's "paying the iron price" for me during the original run, and buying the Blu Ray set when it comes out.