If you look at his per36 numbers, he really isn't far off from the production level of his prime. If Pop wanted to run plays through him, or needed to, Duncan would still be posting 20+ and 10+ a game. He can do it. But Pop knows that the success of the team depends on the depth of th team, and so sets up an offence that gets everybody involved.
Duncan can easily play until 40, and if he was as much of a glory-hound/ball-hog as some other starts who have been in the league for 15+ years, he could be averaging 25 points a game (and he could have been average that or more since his sophomore season if he made an issue out of getting the ball the way some people do).