As others would point out, he wasn't a terrible coach. Early on, the team played hard and overachieved given the severe lack of talent around them. As a GM, it was a different story.
However, I always come back to this: how much of a coach's success is really situational? If we had gotten Duncan in 1997, would we eventually have won a couple of titles with Pitino and would we all be sitting here talking about how happy we are that Gaston has the balls to hire Rick Pitino and give him the Presidency?
(On another note, in such a universe, Antoine Walker's fate might've been much different playing next to Duncan from his second season in the league.)
I'm not entirely convinced that would've happened, but I think there was a chance.