Just to clarify what I said above, I agree that Laver DOES have a GOAT argument.
But...I still don't think CYGS is, on its own, "the GOAT marker." It is certainly the Holy Grail of tennis accomplishments, but we still have to consider everything else.
I mean it is just a hypothetical, but imagine if Daniil Medvedev wins all four Slams next year, then decides he wants to become enlightened, quits tennis and joins an ashram. Does that make him the GOAT? Of course not, although it would make him very interesting and with a certain mystique, not unlike Borg retiring young.
Actually, Borg is another good example. He retired at 25 (for all intents and purposes) with 11 Slams and great statistics because he never played while in decline (unlike Federer, for example). We can play the "what could have been" game and consider him arguably the greatest of all time, imagining him playing another five years and winning tons more Slams. But even putting aside the fact that Mac had gained the edge over him, we can't credit him for what he didn't do. I think he's a unique case, because his ability is more than his "mere" 11 Slams, but on the other hand we just don't know what the rest of his career would have looked like.
(My personal guess on Borg is that he would have have still been really good for a few years, but not the player he was in 78-80. I'm not sure he would have ever been able to regain the edge vs Mac, and further, that Lendl would have caught up to him, especially once Ivan hit his stride in 84-85. He might have taken a Slam or two away from Wilander (RG 1982-83), and maybe won Wimbledon once or twice more, but would have hit a wall by 1984 or so at age 28. So my prediction for "what could have been is maybe 3-4 more Slams for Borg, finishing with 14-15. Maybe one or two more if he decided to sneak over to Australia and take one of those easy ones from Kriek (although then Mac, Lendl, and Connors would have followed to try to stop him).