That would be a really convincing argument if Duncan hadn't actually been running all over the floor and in foul trouble without your imaginary matchup problems. Duncan wasn't covering Howard, yet he still had to chase him around. It might have been nice for someone to have been standing in the lane able to defend the rim against the layup drill.
Since Duncan is such a defensive liability that you would put him on Dampier or Diop because he fouls out in fifteen minutes against Dirk, why would you have him as the only big on the floor? Do you really respect Diop or Dampier's offensive game enough to put a hall of famer on him at all times? If Duncan can't freelance off of one of those scrubs then he should probably never be put in a game again, and if the Spurs are going to lose a seven game series, it's not because they were lit up by Diop or Dampier. They are one trick ponies on the offensive end, and they are much less likely to hurt you on putbacks if Rasho or Nazr are in the game.
Since only you and the Spurs coaching staff believe that Josh Howard is beating anyone with his jumpshot, putting a small defender that can't stay in front of him is a great alternative to a having a big defender that can't stay in front of him. Of course, keeping the big defender at the rim to block or change his shot wouldn't make any sense, because Josh Howard is going to beat you with that jumpshot that was so effective against the Heat. The only alternative if you are the Spurs is to mothball both of your centers and play a style of basketball that is completely unfamiliar to you because you are so afraid of Josh Howard's jumpshot and Erick Dampier's offense on a team that finished second in the division to you.