The problem is even if Square Enix got their collective fecal matter together and produced a really top-notch game, there's still every other video game developer making their own games and a bunch of them are also going to be great games as well. More games are being produced than ever before so it'll be something special for a Final Fantasy game to become the defining game of a console generation. Usually there's quite a few contenders for that crown anyway.

I can see a Final Fantasy game being famous within the series, like "the best Final Fantasy since FFVII" (or whatever game you thought was best) but it's difficult to see it being called "The most impactful game since FFVII". A lot of games can be said to 'redefine' the genre.