Korchnoi plays the French, but it is not his primary weapon against 1. e4.
I know of no player who's name most people would know who employs the French against 1. e4 as his or her primary weapon. Even the best French players, like Shulman, use it as an occassional weapon not as a primary opening choice.
I'm surprised you haven't heard of Alexander Morozevich. He plays the French almost exclusively, and is rated just a smidge under 2800 FIDE. (2760 last I checked).
I've used the French to beat an International Master in standard time, and several NM's and FM's in online blitz, so I don't think of it as a poor defense. Perhaps in the Super Grandmaster field it gets harder to eek out a win, but it seems strong enough to win up to the 2700 level at least.
So maybe the French wins in 2700 or less. And after that perhaps flaws begin to emerge. But even then, I say "maybe." If there is a refutation of the French, I've yet to hear about it.
It is inferior to Caro-Kann, Petroff, Ruy Lopez, Sicilian and Pirc.
Even if the first ones are vaguely something I could believe if you convinced me enough, not the Pirc (which btw has never being a main weapon either).