Well, ASOIAF hasn't been completed yet, and I certainly haven't seen ASOIAF with the serial numbers filed off being published yet, as was so clearly the case in a number of bestselling series with regard to LotR, so I wouldn't say he's already done it. I'd go so far as to say he can't "have done it" since he hasn't completed the series yet and it's still possible that every book will be considered horrible from this point on, and ultimately the series just remembered as "those first few books were so promising" but overall the series as a whole remembered as garbage - already lots of people are saying bad things of the last two books. It could be another WoT where people had a reason to like it at first but then stuck with it out of sheer stubbornness and refuse to admit it's worthless, on the whole, merely to convince themselves that they didn't
totally waste all those hours reading
garbage.
There was a clear trend towards what folks call "gritty" fantasy in the 90's from several writers, not just Martin. With the HBO series especially, Martin has now become synonymous with Fantasy, much as LotR/Tolkien was previously synonymous with Fantasy. Once ASOIAF is actually finished, in 20 years, and we start seeing what are ultimately variations on the exact same characters and the exact same plot being reprinted as though they're something so astonishingly new and a horde of fanatic followers swearing that those authors are the greatest thing since Jesus, I mean Martin, then I'll say we're definitely there. But we're not there yet.