How is that most likely?
He's only 26, and has a vice grip on the French Open that almost guarantees him that tourney every year because of his dominance on clay.
Roger turns 31 next month. He probably only has 2-3 more years of chances unless he bucks history and doesn't age.
I think Nadal will pass him unless someone else figures out how to beat him on clay. He has already won 7 of 8, and will probably win another 3-4 of those alone before he's done.
The fact Federer can't beat him at all anymore at majors also complicates things because he is dependent on someone else knocking him out if he wants to add to his major total.
I like Federer more, but the way it lines up Nadal has a really good shot at ending up with more majors at this point thanks to the French Open.