After the first encounters with the Panthers the US tankers wanted as many 76's as they could get. There was even better ammo available from the TD's. So from the soldiers point of view, the 76's were better. And, if the idiots had not decided to down grade the gun by wacking off a couple feet!?!?
Imagine the result if the procurement offices were not so heavily staffed with artillery officers. These men considered tanks toys, and the "big guns" the rulers of the battle fields. Their pets were the TD's, and they went to a lot of trouble to make sure their pets did not have any competition. Remove that short sighted view and a new tank similar to the Tiger might have been available in Normandy. Or, we could have been mass producing a 17pdr in a properly designed turret.
I don't think that is true at all. Tanks were considered exploitation vehicles, not tank-killers and certainly not "toys". The 75mm was still an excellent TANK gun under that doctrine. It just couldnn't kill Panther's frontally. But how many M4 gunners ever even had a Panther in their sights? I guarantee it is a small minority. Remember too, even the 3" guns used by the tank destroyer battalions proved absolutely inadequate against the Panther in they July fighting. The 76mm gun, with essentially the same AP performance and inferior GP capability was not really an answer.
There was a failure to foresee future requirements across the board. The M4 for example was designed to a 1941 requirement which recognized the 3.7cm PAK36 as the standard infantry anti-tank gun, thus the M4 had armor to protect against this threat. Nobody seems to have given much though to what anti-tank guns would be in 1943-44. Just as development of the M4 started the same day the M3 was ordered into production, so to IMHO should plans for an M4 successor begun in earnest far earlier. 1945 was too late. The tank destroyer doctrine had proven faulty already by the end of 1942. If the lessons had really been absorbed perhaps the TD battalions could have been eliminated as they should have been and that effort devoted to an earlier M4 replacement. At the very least TD's should have had 90mm guns from 1943 but that was another failure to foresee requirements.
In a really ideal world the T25 with a 90mm gun would have replaced the M4 in production by the spring of 1944 (if not earlier) and TD battalions would have been reorganized as tank battalions and assigned directly to infantry divisions. But then, this would have totally upset the invasion plans. The 76mm wasn't really an answer because it reduced the tanks ability to do its primary job. The 17-pounder was certainly NOT the way to go as it was even worse in that regard. It didn't even have a HE round until the fall of 1944 and tanks fire far more HE than they do AP rounds against tanks. The 90mm gun resolves all the firepower issues and using the T25 keeps it packaged in a tank that maintains the M4's advantages of strategic transportability and tactical mobility. The T26 I think was a mistake. The extra armor was not enough to grant it significantly more immunity from German anti-tank weapons while the extra weight was a significant handicap. It didn't become viable until more powerful engines were available post-war.
If nothing else, the German's had a HEAT round for the 7.5cm PAK 97/38 which had proven far more effective than conventional AP rounds against tanks. Had the American's copied this ammunition and had it available in quantity by the invasion of Europe it perhaps could have at least eased the strain. I am not proposing it as "the solution" naturally, because that would have required a new tank and gun.