We all know how successful the T-34 was during and after WWII: not only did it undergo a lengthy evolution in its own right, but later directly led to the T-44 and the ginormously successful T-54/55 and its international derivatives (and by extension T-62, etc). This isn't even taking into account the SP guns/tank destroyers based on the chassis. So, why wasn't the same done with the Sherman?
The humble M4 ("the T-34 of the West") was every bit as good a medium tank as its Soviet counterpart (slightly less mobile but slightly more reliable) and it too underwent lots of refinement during the Second World War (and post, in the hands of other nations like Israel). Why did its official development stop in 1945?
It shouldn't have been all that hard to drop the height of the tank as was done with the T-44/54/55 (IIRC, the height of the Sherman was necessitated by the radial aircraft engine, so an engine redesign could have enabled this), increase the thickness/slope of the armor, widen the turret ring so it could accomodate a turret with a 90mm gun like the Pershing/Patton, etc. Such a tank would have been lighter than the Patton series, just as well armed and armored, and probably quite a bit less expensive and faster to build. (They would probably be less comfortable for the crew, but that didn't bother the Russians, did it?)
Was this ever considered and rejected for some reason or was it just assumed that newer-generation tanks had replaced it?