I just think LeBron is on the cusp of being the best player in the game. if you switched Pierce and LeBron next year (while I agree we can't really know this) my guess is the Celtics would have the best record in the league and the Cavs would just make the playoffs.
In my mind, Pierce's greatness needs others around him to allow him to use his talents while James is simply a dominant player capable of greatness no matter who is around him.
It doesn't diminish Pierce to say he's one of the top 2 small forwards in the league. I'd say he's one of the top 10 players in the game:
in no particular order: Kobe, LeBron, KG, Duncan, Howard, Pierce, Nash, Paul, Dirk, Williams