The issue of the Sherman's 75mm cannon was one of politics. There were US observers that reported on the Invasion of Poland & especially the Battle of France. They developed a doctrine explicitly to deal with massed armored attacks & the Tank Destroyer corps were born. The US had the 76mm M1A1 as early as 1942, and the gun could be fitted in the smaller turret that housed the 75mm cannon (loads of Sherman -E9s postwar show this). However the installation was rejected as it was felt the turret was not big enough to properly work the cannon, the 76mm threw an inferior HE shell, and more importantly in 1942-43 the 75mm was sufficient to penetrate the 50mm armor of the Pz III & IV operating at the time...
Of course nothing is static & the Germans were upgrading the armor on their tanks to 70 & 80mm respectively, but nonetheless dedicated anti-tank work was the job of tank destroyers so if a tank showed up the Sherman couldn't deal with, call for support...
But the idea that the Sherman was just an infantry tank is a bit of a misnomer IMHO. THe British called it a Universal Tank, & Shermans were used just as much supporting infantry in independent tank Bns, as well as being the primary tank equipping Armored Divisions, whose glory job came in exploiting gaps in the enemy lines. The Germans were surrounded in Falaise in part because of reliable, fast Shermans were able to exploit the breakthrough at St Lo & surround the Germans in a pocket.
Finally I want to stress ultimately how important the strategic utility of tanks are in dictating the tactical situation at the cutting edge of the battle. After all, a Tiger is a fearsome tank, certainly, but if you have insufficient concentration due to breakdowns & losses due to scuttling, then that is going to have a strong impact on the tactical options German commanders will have on-hand.
Damon.