During OTL US tank development progressed from the pre-war M2 forward with the stop gap M3 and finally the M4 Sherman.
With the M4 Sherman the allies had a tank which was in 1942 on par or better than the bulk of the German armor. As such it made some sense to mass produce the Sherman in preparation for dday.
The development continued eventually arriving at excellent designs such as the easy eight.
Given this history, what would be required for an earlier replacement of the M4 Sherman as the allied standard tank.
Would it be as easy as abolishing the existing tank destroyer doctrine?