Well, the publication dates for Vol 1-3 tell a radically different story about the effect of finances on an author's discipline:
A Game of Thrones - August 1996
A Clash of Kings - November 1998
A Storm of Swords - November 2000
This is the publication history of the first three novels in the series. What isn't mentioned above is when sales started to REALLY ramp up and take off. That came in the wake of A Storm of Swords.
I bought AGoT in its paperback edition, so I am relatively certain that was in early 1997 that I got it. I bought all subsequent volumes in the series in hardcover more or less on the first day of release. Thing is, with ACoK and ASoS, that wasn't initially bestseller territory. There were fantasy fans at the time in 1997 who were huge fans and it won some major awards. But the word of mouth at the time was literally that: Word of Mouth. The Internet did not play all that large a role in the popularity of the series in the late 90s at all. Netscape and all the rest of the early web was still very much new in the late 90s and had not yet become a defining feature of modern society that it would take on in the early 2000s with the spread of cable internet.
Still, all of that changed for GRRM with ASoS. That novel is so exceptional that it elevated the two books before it and sales of the series as a whole started to take off as the novels moved out of the genre and into the mainstream. ASoIaF became a monster of a series in the wake of ASoS, one that would ultimately have a real and lasting impact on the genre and popular culture itself started with ASoS. Popularity of the novels ramped up significantly after that and sales started then moved exponentially.
The result was that GRRM no longer had to worry about paying the mortgage. He started to make damned good money off of those three books. And that's when the slowdown starts. A Feast for Crows was not released until 2005; a disappointment as many of the main characters do not appear with their own perspective chapters at all, notably Jon and Tyrion. A Dance of Dragons was released at the end of the broadcast of the 1st season of HBO's Game of Thrones, and sales of the whole series increased by orders of magnitude, world-wide. GRRM had more than HALF the top 10 best-sellers on Amazon for more than a year based on the phenomenal success of the HBO series. By that time in 2010, GRRM was already wealthy enough that his attention span necessary to write had waned; and with the sales of his novels after the 1st season of GoT went viral, basically, GRRM became stinking rich off his per copy royalties; indeed, the author himself became a celebrity. That simply doesn't happen to fantasy authors. Yet it did, just the same.
And so we have not seen a new novel in the series in 12 years - and counting. Turns out, having to pay the rent is an excellent motivator for an author.
None of that should take away from the fact that ASoS truly is an outstanding candidate for the Best Fantasy novel of all time. It is an achievement in fiction that, because of GoT, all of this became part of popular culture: The Red Wedding, the attack on the Wall by the wildings and the death of Ygrette, the poisoning of Joffrey, Oberynn's duel with the Mountain at Tyrion's trial, the strangling of Shae and Tyrion's crossbow bolt into Tywin as he sat on the toilet? All of this is ASoS.
The only novel which can compete with the footprint left by A Storm of Swords is The Lord of the Rings or perhaps Harry Potter (as its sales were literally unprecedented). That is where A Storm of Swords rises to - that's as rarified an atmosphere in fantasy literature as it gets.