Well, Game of Thrones is a bad example because it could really only exist on a network like HBO. Half the scenes have naked people in them.

But the point isn't that it would be impossible for shows like Game of Thrones to gain mass ratings, but that it is harder for them to do so. How many other popular shows like GoT are there versus ones like Law & Order? It's a more uphill battle because for unique shows because people instinctively like what is familiar to them. Cop Shows are a very familiar part of American culture so it's much safer and easier to make a show like that than one like Sherlock or GoT. So that's what Elementary has done and why it has a chance to succeed.

In the end people generally will watch a show if it is great TV, though not always (Firefly and Arrested). But if a show isn't great (ie. is mediocre), it has a much better chance to succeed if it is some derivation of the buddy cop formula versus something fantasy based like GoT. The evidence (all the cop shows currently on air versus fantasy shows) certainly points that way.