Rafa can win 4 more French Opens to get to 17, and he wouldn't be the GOAT. He has to have more balance than that. You can't just utterly dominate one surface but merely be well-accomplished on the others and be the greatest tennis player of all-time.
2 US Opens, 2 Wimbledons, a 1 Australian Open just isn't enough to compliment 12 French Open titles when you're comparing a guy to a man with 7 Wimbledsons, 5 US Opens, 4 Australian Opens, and 1 French Open.
1. Nadal is going to win more non clay slams. You are living under a rock if you dont believe this.
2. If Nadal did reach 12 French Opens it would just indicate Nadal's greatness on clay is so profound, that it is worth as much as Federer's greatness at Wimbledon and the U.S combined.
You can think what you want and continue to post your 25 times a day saying "Fed is GOAT no matter what, Fed is GOAT no matter", but in the real World Nadal will be viewed as GOAT with 17 slams no matter how he gets there. Maybe even if with 15 given his mammoth head to head ownage of Federer, which btw even favors him excluding clay (3-2 in slams, 8-7 overall), his greater longevity at the top, his winning more on any surface than Federer could on clay (which throws out the so called one dimensional argument btw, Nadal on grass or hard courts > Federer on clay).
Will he be the GOAT mostly due to his clay greatness, especialy in your proposed (never going to happen as Nadal is going to more majors off clay contrary to your beliefs and wishes) scenario? Yes. Clay is part of tennis though, and that is just the way it is, like it or not. A swimmer could become the best swimmer ever over Michael Phelps just by winning enough freestyle golds if they were that great. Say someone who won the 50, 100, 200, 400, 1500, Open Water, and all 3 relays at the next 3 Olympics, they would be greater than Phelps, regardless if they swim no backstroke, breastroke, butterfly, or individual medleys, and Phelps wins gold medals in all of those except breastroke. Now that said if Nadal had nothing on other surfaces could he best ever? No, but that ship has already LONG sailed. He is already a multiple U.S Open and Wimbledon Champion, with the Career Slam, Olympic singles gold (not on clay), soon probably to reach double digit hard court Masters, many other finals and top results, so unfortunately for Federer fans that argument will never be able to be made.