The problem was it was the decisions made in those 2.5 years and the decisions made in the three years prior that determined what the problems in the infantry replacement policy were. Fundamentally, the Army operated in the dark and made best guesses on what should be done, based upon what they saw from the outside looking in at a world at war.
From the beginning, the Army prioritized airpower and technical means - armor and mechanized forces - over infantry. One consequence is they simply assumed what the distribution of casualties between the various arms would be and ended by grossly overestimating the replacement requirements of armor and mechanized forces over infantry. They also assumed that Infantry could get by with the least qualified personnel, because the more technical arms would shoulder much of the burden in combat. That was also the rational behind the ASTP - a more mechanized and technically oriented force would require a reserve of intelligent and highly trained personnel to man it, so better to pick the smart ones and educate them as much as possible in advance.
Another unintended consequence came from the decision made immediately after Pearl Harbor to accelerate the rate of activations by drawing filler personnel for newly activated divisions from Selective Service Reception Centers rather than from Replacement Training Centers, effectively making the new division cadre responsible for the 13-week basic training program. To facilitate that, most of the RTC were temporarily shuttered and personnel transferred to become cadre.
That was fine so long as divisions remained in the States training, but once deployments to combat began and casualties began mounting, it threw the system out of whack as the divisions in training became reservoirs of manpower for the divisions at the front.
Add in another decision such as that made in mid 1942 that units could not deploy from the Z/I with 18-year old personnel. They had to be an arbitrary 18 1/2 years old to POM, so every division slated for overseas began shedding personnel to other units, while stripping personnel from other units. Personnel turmoil became a hallmark of American organizations.
Its possible to go on and on, but fundamentally it was lack of experience in mobilization and mobilization planning, lack of experience in modern war, limited numbers of qualified officers and NCO, lack of facilities, and so on.