I agree that Mozart was revolutionary, probably mostly through influence rather than directly, but just to play devil's advocate:
If the piano had actually existed earlier, I'm sure plenty of composers would have seized upon it and invented the piano quartet before he just happened to write one. Also, Gluck's Operas are still performed and enjoyed. Also, he couldn't write fugues, by his own admission (The less said about that organ piece, the better). Then again though, no-one's really been able to do that since Bach, nor have they really wanted to unless as a gimmick, exercise or 'showy-off' thing.
