Major titles are obviously very important, but I totally disagree with the notion that Rafa aboslutely must match 17 to be the GOAT.
Here are a few of the problems I have with that notion (in no particular order):
1. If the numbers of Major titles are the only measurement of the GOAT, it would mean that until the early 2000s Roy Emerson was the GOAT. We know he simply wasn't (and probably wasn't even in the top ten at that time). If you want to only look at the Open Era, it would mean that Sampras automatically (without any question whatsoever) passed Borg with his 12th title even though Sampras only has one SF at the French on his resume while Borg, despite the missing USO title, has a resume that shows a fair degree of success on his weakest surface (losing hard court finals to Mac and Connors is not exactly shameful).
2. If Rafa goes on to getting his 17 by winning 4 more FO, his overall numbers would be:
12 FO, 2 Wimby, 2 USO, 1 AO
If, on the other hand, he goes on to get a total of 16 by winning a FO, a Wimby and and AO, his overall numbers would be:
9 FO, 3 Wimby, 2 USO, 2 AO
Is he a better GOAT Candidate in the first scenario just because his overall number is 17? I just don't think so. The second scenario is actually far more impressive to me as it further supports the all-round success feature that should be an important factor in the GOAT determination in my opinion. He already has that point covered enough to be "eligible" for GOAT status perhaps, but to actually get the GOAT title it is still something that needs a bit of improvement I think (even without doing a "Borg" early retirement, Fed beats Rafa by a significant margin in winning percentages at 3 out of the 4 Majors - it's only the HUGE lead that Rafa has at the FO that makes the overall percentage better for Rafa). Having said that, Rafa shouldn't be punished for his FO succcess AS LONG AS it is accompanied by non-clay success as well. Obviously the best scenario for him would be a few FO and a few off clay to finish with about 20 Majors.
3. A good set of "context glasses" is the key to the GOAT debate in my opinion. Borg has some historical context points that are not reflected in his number 11 for example. Obviously Laver does too. Conversely, Sampras's 14 has a big asterisk beside it due to his FO problems. On the other hand, Federer's FO "problems" were due to the King of Clay, otherwise he might have even more Major titles, which perhaps is a contextual point very much in his favor. Meanwhile, Rafa had Fed early and Novak now to contend with. And the list goes on. Context is ultimately the key here no matter how "objective" we try to make the basis for the determination of the GOAT.
The grass and hard courts are slower now with higher bounces, for one more example. Just how much more impressive ("greater") is Borg's FO-Wimby combo than Nadal's (or Fed's for that matter). On the other hand, that is countered to some degree perhaps by other factors about Borg's era. But there is no getting around thinking about all of these factors and ultimately making some sort of assessment about them.
It is NOT just the number 17!