​The M4 did not have a wimpy 75mm gun in 1941.

The Ordnance QF 17pdr ​wasn't available until 1943.

Integrating a new weapon into an existing tank design is not what most engineers would advise since the costs can be quite prohibitive.

The thinking in the US was that the the M26 was going to be ready 'any day now', which didn't happen.

The thinking in the UK was that the Centurion 'universal' tank wasn't going to be ready in time, so a LIMITED conversion program was a reasonable accounting risk.

Israelis faced the same choice in the 50s when converting the same Shermans to the M50/51 spec.

Greg