Viktor Korchnoi
Igor Glek
Nigel Short
Mikhail Botvinnik
Note that for many GMs, the same opening is not used exclusively throughout their lifetime, even as their "main weapon".
Take Kasparov vs 1.d4. In the 80s, before he became World Champion, he favored the Modern Benoni. Mid 80s thru Mid 90s, it was the King's Indian Defense. Mid 90s thru the start of the 2000 match, Grunfeld. After Kramnik shalacked his Grunfeld, he never really had 1 defense he stuck to. He played QGA's, Slavs, just about anything you can think of.
So if you look at the players above, and it seems like they aren't playing the French, look at a different time period in their career, like I know Botvinnik had a long stint with the Caro-Kann at one point in his lifetime.
Life Long French GMs? There aren't many. Not even sure if Wolfgang Uhlmann played it for his entire lifetime!
Also, your statement about studying GM games is on the assumption that they are annotated. If all you are doing is running thru database games with no annotations, you won't get far.
Also, better to study on an actual board rather than on a computer. Being forced to move the pieces actually does more for you than clicking a mouse, especially when you go through a side variation and you have to go thru the moves again to get back to the position you were at. Drills the lines into your head doing it that way.
It is said that the best way to study an opening is by going through the games of grandmasters who play it.
Can you tell me grandmasters who used 1...e6 as their main reply to the king pawn opening?
Thnaks