Tennis--and sports in general--is funny like that. A single match can bring the perception of that. I mean, Mac said that the 2017 AO would decide who the GOAT was. Now that's Mac, but he kind of speaks for the hyperbolic and fickle fan in all of us.
Of course that is absurd. Just as you can't say a single match determines whether Novak is the GOAT or not, you also can't say that a single match between Roger and Rafa determines the GOAT.
That said, if Novak had won that match, I think the collective argument would essentially be over. Sure, Roger and Rafa diehards would still claim that Novak wasn't the GOAT, but most could concede to Novak, with his 21 Slams and first CYGS since Laver.
But this illustrates, again, the problem of overly focusing on any specific, or group of specific, stats. I mean, if we say Slams are everything, they the logic holds that Johan Kriek was better than Gerulaitis, Muster, Stich, Chang, Ivanisevic, Roddick, etc etc, which he wasn't. Kriek was a guy who barely made the top 10 and won the the first of his two Slams without facing a top 50 opponent, the second without facing a top 20 opponent. Some ATP 500s--and even 250s--are more competitive.
But the CYGS is special, in a way that, say, winning the Triple Crown isn't. Over the last couple decades baseball stats have evolved, so that most people under the age of 60 know that BA, HR, and RBI aren't the most telling stats of a player's value. I mean, statnerds started focusing on OBP and SLG in the 80s, and the 90s introduced advanced stats like Win Shares, and then of course WAR today.
But the CYGS isn't like the Triple Crown. It is an amazing feat - the big kahuna of tennis. I can't really think of anything comparable in other sports (or at least those that I know anything about).