I agree with you and with McCain. But I'd add that it wasn't just Rumsfeld's errors. It was also the inability of this administration to effectively convey their message or vision regarding the importance of this war.
Was it Churchill who called war a series of blunders leading to victory? We had massive errors in military judgment in the first years of WWII. Iraq is no different. It's perfectly fine for McCain to point out these errors. He's not saying anything he hasn't said before--that we didn't sent enough troops, etc.
Of all the candidates running for the GOP nomination, McCain is the only one (other than maybe Rudy) whom I trust to fight a strong and effective war against the Islamists.
The difference is Roosevelt and Churchill were able to learn from their mistakes. Bush's attitude seems to be to not do anything different until he has no other choice and then give only a token response.