The problem with this line of thinking is: what happens when Lebron (or another superstar), is not in the playoffs. The casual fan thinks Duncan is boring. It's not being a homer saying this is a bad idea in general for the NBA, it's just being observant.
Ask yourself this question: If the league had been promoting Tim like Lebron since 1999, do you the ratings would be so bad?
Look, I have no problem with Lebron, and as a basketball fan, I'm glad he'll be in the Finals so the ratings will be high.
But, I have to say, the NBA misses great opportunities for marketing. ie, 2003, they could have gone into great detail into David Robinson and his story during the Finals. They didn't. Instead, they decided to review summer movies during halftime (no joke!).
In short, the current NBA ratings slump was in part created by the NBA by hyping teams and players that very rarely make it to the Finals.