Two things:
1.) I agree that the whole was greater than the sum of its parts. The team that was there hasn't done anything as good since the band broke up, so to speak. That's true with a lot of artists, though, who often work better in collaborative conditions, but don't get to do as much of what THEY want to do as they'd like (which, in turn, leads to the breakups). Kurtz's accomplishments after the fact may not be legendary, but arguably neither are Lucas'.
2.) Nobody is golden forever. The various directors who could seemingly do no wrong in the 70s and 80s have had plenty of missteps since then. Frankly I don't think Scorcese's made a truly brilliant film since Goodfellas. Spielberg's had some duds with Indy IV, and his various Tom Cruise films. Has Coppola even done anything since Dracula (which, while entertaining, hardly matches The Godfather I and II or Apocalypse Now)? Everyone "loses it" at some point. So, maybe Kurtz "lost it" after he left Lucas, and Lucas "lost it" around the same time.