I got discounted cable by subscribing to HBO to begin with which means that HBO is paying At&t a subsidy in order to subsidize my cable and in so doing I become a HBO subscriber.
No, it doesn't exactly mean that.
You can't actually purchase HBO without some sort of cable. Companies like Comcast have basic TV, internet, and HBO bundles, but they are still very costly. And a decent chunk of that cost is because they have to pay HBO. And when cable companies give deals like a free year of HBO, that's on their dime in order to entice you into multiple years of service. They're still paying HBO for that.
If you're unsure of how HBO makes money, you could read this:
http://www.investopedia.com/financial-edge/0812/how-hbo-makes-money.aspx
According to that article, the speculation is that HBO makes $1.4 billion a year just in subscriber fees. That makes Game of Thrones' budget just a drop in the bucket.
I sometimes wonder if GOT would have lasted long enough to build an audience if it had come around a few years earlier.
Maybe. But it's hard to say if it would have had continued support by HBO. I think they realize their mistakes with
Rome and
Deadwood since they made a boatload on DVD sales for those shows, something that they hadn't really factored in. GoT could have suffered the same fate if produced in the same era.
TV now is changing with so many shows on disc and streaming, that people are flocking to these types of shows and eating them up. Add to that the technology changes both in streaming and for creating VFX, and it's more likely that shows in this decade can reach for that bar and still succeed. In another decade, the bar may even be set higher. At least if television trends continue.