Don't forget the Sherman was a pretty good all-round tank, it fired a useful HE round and its gun was perfectly capable of dealing with Panzer IIIs and IV's. It ONLY fails against Tigers and Panthers and even then they were able to out fight them and it wasn't the case of 5 Shermans for 1 Tiger, far from it in fact.
The WAllies were usually somewhat behind the curve. For the Brits it was hamstrung by the slowed pace of development and then re-armament pre-war and doctrinal issues combined with the Invasion Panic. The US had to build its armoured forces from the ground up and had several competing ideas but precisely zero experience with modern tank warfare, their last experience with tank combat being the US army's FT-17s in 1918. Because of this and other factors the UK and US were usually a bit late with what was basically the right tank for the right time.
Had the Cromwell come out into proper service in late 42/early 43 with the Comet entering service in Summer 44 then the British would have had tanks that could challenge their German rivals, and the Comet was easily a match for the Panther. The US dedicated itself to churning out the M4 in huge numbers having evolved from the M2 then the M3 into the M4 which was a good tank, but if they had put it out with skipping the M3 Grant/Lee family then it would have been a world beater.
Also don't forget that tank combat in the West was an entirely different beast to that in the East. There wasn't the big open stretches of largely empty terrain for long range guns to really work their magic, that only happened in North Africa and then it was about defensive and repost style battles. I honestly don't think that a US heavy tank would have made much of a change. The M6 by all accounts wasn't much of an improvement over the Sherman, and had the same gun and later a longer 76mm gun, so its little in the way of improvement.
These are good vids, probably oft linked but still worth watching.