Great point. I have often thought about the apparent futility of armor throughout history and had kind of a revelation.

The purpose of armor is really not to offer immunity from a similarly armed opponent. Except for brief windows of invulnerability offered by advances in technology, there has always been a corresponding forward leap in the power of weapons that make more armor too heavy or otherwise impractical. So the value provided by armor in warfare really is to maximize the expense and minimize the quantity of the enemy's weapons that can hurt you, and increase the level of his skill/performance required to achieve success.

So whether it's tanks, dreadnoughts, or mail shirts armor offers limited protection at certain ranges and against glancing blows but the idea of an 'impenetrable target' in warfare has almost always been the rare exception rather than the rule.

In the case of these Shermans then, no surprise that they were knocked out by high-powered, expensive, and relatively small-numbered German AT guns...they were designed to perform a particular function on the battlefield with minimized risk from cheaper and potentially more numerous threats like artillery shrapnel, heavy MGs, 37mm and 50mm AT guns. Unfortunately for their crews, in the two years that it took to ramp up for the invasions of Europe the Germans had leapfrogged ahead in both weapons and armor thanks to their bitter experience having been leapfrogged by the USSR.

IMO the fault of the Sherman was not so much its level of protection as how long it took the Allies to increase its offensive punch in order to decrease the usefulness of armor on the Panther and Tiger tanks (for all of the doctrinal reasons that have been discussed to death).