I have no problem with calling Duncan the Greatest PF of all time for the time during which he has actually played PF.
If he wants to blame anyone for being called a center, let him blame RC for assembling the team and Pop for choosing the line-up that has clearly had him playing more as a center and less as a power forward in recent years.
It went too far for me when the Spurs whined and got him moved to PF on the NBA All Star ballot so he would not have to compete with Yao Ming for a starting spot. Before that, it was simply semantics.