Thomas Blanton has been critical of every Republican Administration back to Regan. Although I respect much of what he has done in garnering the release of formerly classified material, he has a left leaning bias none the less. While he readily comments on the declassification of POLO STEP you are hard pressed to find anything from him regarding the declassification of VENONA which is curious when you consider how active he was in getting the information declassified, well curious until you realize that VENONA showed that the accusations made by McCarthy during the cold war weren't entirely unfounded, among other things clearly showing beyond doubt that Rosenberg passed atomic secrets to the Soviets.
The quote you paraphrased regarding troop numbers is from a book written by two authors both of which have been (and one still is) on the payroll at the New York Times.
Basically what you have here are liberals making 20/20 hindsight comments about plans made by CentCom to brief the president. You failed to note that these were a base from which war plans originated, not what we went into it with.
In the end, Secretary of State Colin Powell and British Prime Minister Tony Blair persuaded President Bush that the U.S. needed to go to the U.N. to try to legitimize the invasion. Diplomatic efforts over the next few months allowed more time for war preparations and the final option embraced by Rumsfeld - Lt. Gen. David McKiernan's Cobra II - was closer to Generated Start, the original plan, than the various iterations that were subsequently developed and are reflected in the declassified PowerPoint slides.
I'm sure CentCom has vague military plans drawn up for attacking every country in the world, and they are just that vague base plans needing modification based on real situation. Your conclusions (and those of some of the commentators in the article) that this is some sort of smoking gun with absolute proof place far more importance on these documents then is due.
The reality is that we could have and should have been better prepared, and our government and the military were operating off limited information. At the onset of this war we didn't even have any CIA agents or informants on the ground in Iraq, the CIA quite literally had to go on a major recruitment project in the months leading up to Iraq just to start getting informants. As it was even in the initial stages there were blunders, including a preemptive strike to take out Sadam before we even started an official military campaign. Two days before "Shock and Awe" was commenced US fighters dropped bombs on a farm where Sadam was supposedly hiding based on misinformation from these quickly recruited informants. It was even reported to the President that the operation was a success and Sadam had been killed. This was not the case, and the action gave away an element of surprise.