HBO is in a good position right now, especially considering the onslaught of shows from Starz and AMC and the like. GoT is a big part of that. They didn't really used to have that competition, so they are happy they're doing so well now. The show is making them money, and they increased the budget 15% in the season just for that big battle. They put money into it and they get money out of it. If it's like The Sopronos, or that show with the sluts in NY, then it will just keep giving back the money for years and years. If people somehow lose interest, then they'll drop it without a second thought. Viewership has been steadily climbing, and it's kind of a show that you have to invest some mental power into to keep things sorted out. If you can get people to invest in something like that, then they'll stay around to see what happens. Look at Lost, even when it went to shit people seemed to stay with it. GoT actually has a conclusion figured out.
Look at this shit from wikipedia:
Game of Thrones: Viewers per episode (in thousands)
First airings only. Repeats and On-Demand aren't counted here.
Blue = season 1 ratings
Red = season 2 ratings
(That big dip in season two is Labor Day weekend)