Fed was a better player on clay in 2005-07, he didn't improve since then, he's only taking advantage of the other top seeds going out so I don't feel this French Open will change his legacy. We already knew he could beat these players but could he beat NADAL and DJOKOVIC in the French Open? The answer is NO.
Nadal won Wimbledon and the Australian Open because HE ACTUALLY IMPROVED on grass and hardcourts and he beat the best to earn his titles.
If Fed wins the French Open it will be because others were upset, NOT BECAUSE HE IMPROVED ON CLAY TO WIN THE FRENCH OPEN.
The key for Federer to be the GOAT is not the French Open, it's Rafael Nadal, the man who beat him in the Australian Open, Wimbledon and the French Open finals in the last 12 months.
Nadal and Federer have played 8 matches in Slams with Nadal leading the series 6-2. This is exactly what Federer needs to overcome to be the GOAT, not a French Open with a depleted field.
I agree with your conclusion but I disagree with your arguments.
Even though Federer won the FO he won't be the GOAT in my opinion. Tilden, Gonzales, Rosewall and Laver are superior in my mind.
But if Fed won RG he will show that he is currently the best on clay.
I agree that he had not improved on clay
but
NADAL HAS REGRESSED ON CLAY
and DJOKOVIC has never won any 5-SET MATCHES EVENT AGAINST TOP HEALTHY PLAYERS AND EVEN LESS ON CLAY (he won only once such an event, the 2008 Australia Open against an ill Federer but not against a healthy Fed).
So if Fed won RG this year he will be without any doubt the best claycourter of the year (remember that he had already won Madrid).
However, at the moment I write that post, Friday morning, Roger hasn't won Roland (he hasn't even played his semifinal against Del Potro) so wait and see.
Here I explain why Nadal regressed that spring :
the probable main reason why Nadal lost Roland that year is not that he was physically or mentally tired (and he was) or that Söderling played great (and he did) but that the 2009 Nadal's forehand on clay is not effective on high balls : it's possibly a technical (and not mental or physical) failure.
Patrick Moratoglu (the man who owns and manages a tennis camp at Grignon, a few miles from where I'm living, near Paris, and who trained there Baghdatis a few years ago) felt since the very beginning of Garros 2009 that Nadal wasn't at his best in particular on the forehand.
L'Équipe, the French sports paper, lent him some videos and photos of Nadal in 2007, 2008 and 2009.
These are very clear (in L'Équipe are published one photo in 2007 and another one in 2009) and let Moratoglu see where Nadal's weakness was that year.
Nadal's 2009 preparation is very lower and shorter than his 2007 or 2008 one when he used to began his forehand loop much higher and then got a very better follow-through and power. You can clearly see that in 2009 Nadal began his stroke at knee's or hip's height and so the backswing is very shorter while in 2007 or 2008 his arm was very higher and the preparation very longer. So last week Nadal was really annoyed by high balls on his forehand (and Söderling exploited it to the full) while it wasn't the case in the previous French champs.
Last year or in 2007 Nadal could also do his attacking "uncrossed" (I don't remember the English term : the opposite of crossed) forehand with superb angles while he didn't (and possibly couldn't) in May 2009. That year his forehand was mean and not a weapon at Roland as years before. In 2009 his attacking play was really less good than in previous years at "La Porte d'Auteuil". And a Nadal who doesn't effectively attack isn't a unbeatable player though he is still a superb defensive player.
So in 2008 and early 2009 Nadal has improved on every "modern" surface (nowadays true fast surfaces don't exist anymore) but that spring he has clearly declined on clay (though he did, he had been able to win Monte Carlo - Barcelona - Rome)
And yes Federer could have beaten Nadal and Djokovic that year because Nadal had declined and Djokovic has played too much and probably will have all his life physical weaknesses. I don't say that he would have certainly won but his chances that year would have been pretty fair against those players.