I don't really see how. Virtually everyone loves Game of Thrones as it is, in spite of its complexity. I suppose it's possible that virtually everyone who likes complex TV already has an HBO subscription, but I find that highly unlikely, since HBO viewers are a small fraction of the TV viewing populace. I don't see why it would be particularly difficult for such a show to hit 13 million viewing figures on a major network. As I said, when you count all the airings it already gets ten million viewers an episode on HBO. Granted, HBO airs episodes several times, but it strikes me as colossally unlikely that a network wouldn't be able to best those figures on first viewing. Ten million viewers an episode is an average, and the pilot of this got, what, thirteen million? It's highly unlikely it will keep that many viewers. It'll probably taper off to about eight to ten million.

So yeah, don't see how it's unlikely for shows like Game of Thrones to get similar viewing figures on networks. For it to get the same viewing figures as shows like Law and Order? Yes, that is a bit more unlikely. But it's a bit more unlikely for an average show to get that kind of viewing figures too. There are plenty of shows like Law and Order, or American Idol. Most of them are nowhere near as massive as Law and Order or American Idol.