Well, as Chieftain's video showed there was in fact a couple of 30 ton Sherman "replacements" that were explored. The M7 even had a factory built for it. But most turned out to have problems and adding a big gun always made the turret too cramped.
I certainly wouldn't just slap a larger gun into the standard M4 turret. But neither the T23 (i.e. M4 76mm) or M26 turret were particularly cramped, and both would fit on an M4 hull (though the M26's extra armor thickness made that probably a bad idea weight-wise). And, again, I don't think a replacement tank should have been that high a priority. They should have been working on it (because what else are US tank designers going to do?), but crash programs to get
something out were clearly not worth it. But without diverting attention into absurdities like electric transmission and an autoloading 75mm (much less new awful tanks like the M6 and M7), Ordnance probably could have introduced new useful features like wet stowage and HVSS earlier. Just moving nonessential items to external stowage (as in British practice) and lowering the hull height because the Ford GAA had the driveshaft much lower down than the Wright radial could have gotten much of the benefits (smaller armor envelope) of rear drive while leaving the actual tank mechanically identical. The M24 had the same powertrain as the M5, but while the M5 hull was tall enough to accommodate a radial engine because its M3 forebear had one and nobody had bothered to redesign it, the M24 could retain the same proven powertrain (actually simplified by replacing the automatic transfer box for the two engines with a manual) while lowering hull height significantly. The same hull redesign could've been done to the M4, without even having to try developing a new rear drive transmission. Identical mechanics, smaller target, lighter tank. Seems like a reasonably decent idea to me.
More anti-tank training would have probably helped - a big problem in Normandy is so many tankers were going into action for the first time on the US Army and British side - but the TDs themselves had a number of good and oft-ignored features that made them much better at anti-tank warfare than the normal tanks; that keeps getting ignored because people are too focused on the gun and armor.
The combat histories indicate that tank battalions that got a few weeks with an experienced TD battalion to show them the ropes showed marked improvements at killing panzers. As for TD design features, if the TD branch is never created as I and apparently you think would have been better, they were presumably features that could (and should) have been incorporated either into a revised Sherman or a replacement. Aside from, like, an open top for more visibility. Though the British late-war cupola that could raise itself a few inches to let the commander look (as well as open normally or view through periscopes, of course) might have been worth looking at.
So you slap a couple extra tons of armor on a tank. How much good is that going to do? The Sherman already offered really good protection. You start messing around with a well designed machine and who knows what you are actually going to get.
The US lost 100k GIs in Europe and lost 1k tankers. If you think that slapping another 10mm of armor on a tank or giving it more anti-tank capability is a priority you have it backwards. You need a more formidable platform up to a certain point and the M4 is a pretty good assessment of where that point lay. I'd even say the M4(76) was slight overkill. The M4E2 was a specialized vehicle with severe limitations (overloading the suspension basically) that was customized for a specific role (leading road convoys and assaulting stationary fortifications) and in no way appropriate as a general vehicle. The question needs to be what do the GIs need. The GIs need a tank that can traverse terrain. They need a tank that will spot it's target (not necessarily a tank) and start shooting quickly so the guys outside the tank aren't taking fire. And the GIs absolutely need a tank that will make it onto the battlefield.
This talk about slap on some more armor, give it a bigger gun then up the horsepower to compensate is exactly what led to the Panther being such a dud on the battlefield. Sure it looks great on paper! But the tank that existed on paper never existed. The Sherman on the other hand was the same tank in real life as it was on paper. It actually went cross country like it was supposed to. Trained gunners could find and attack targets as was envisioned. You start messing with that design and hey maybe you save a few dozen tankers but you kill a few hundred GIs or more who are left without the tank support that could have protected them. Or maybe you mangle the design by fixing what isn't broken and you kill more tankers and more GIs.
Please read my posts more carefully. I am specifically
not arguing that thicker armor or more engine was needed. I am saying that a hull designed to pass a driveshaft off the middle of a radial engine under the turret basket to the front is inefficient and could be improved, particularly since the US Army switched to an inline engine that didn't need that extra hull height anyway. Moving nonessential items to external stowage so they didn't take up space inside the armor would have been good. And that since they replaced the hull, turret, engine, suspension, gunsight, and main gun of the Sherman from the M4A1 to the M4A3E8, perhaps
also replacing the transmission wouldn't have been much of a stretch. What are you advocating, that in early 1942 with the M4 complete, US tank R&D should have just taken a nap for the duration of the war? The M4 was going to need replacement
eventually, why not start work in 1942? It's not like the first completed design had to be chosen to immediately become the new standard.
The first tank in the T20 series to get significantly thicker armor was the T26, and I already said that it was a waste because a 45 ton tank in 1944 was not generally worth the sacrifices (with the possible exception of the IS-2). I am saying that
if a replacement was issued, it should have comparable weight to the M4A3E8, and that its closest T20 series analogue, the T20E3, was noticeably
lighter. Remember, identical turret to the 76mm M4. Same engine, same suspension, same controlled-diff steering, identical armor layout (except that the T20 was physically smaller). A tank with the same engine, steering, and suspension that physically weighs
less is not going to have reduced mobility. The primary reason I suggest the 90mm was worth the small additional weight was for the increased HE capability (also in high demand, particularly by the infantry), the fact that it
also provided greater armor penetration is just icing. Certainly by the time US forces were encountering Panthers even occasionally, the panzer forces were collapsing and it no longer mattered all that much.
As for the M4A3E2, A) combat reports indicate that it was very popular and not that unreliable if driven carefully and B) it also used the older VVSS suspension. HVSS or torsion bar would have provided significantly more weight capacity. And it was probably
too uparmored. A more reasonable improvement was probably worth at least looking into. For that matter, the more interesting armor kits developed (but not fielded) by the US were spike or plastic armor designed to add protection against Panzerfaust/schreks. Not slabs of armor plate to bounce, like, 88s. It is not weird that tanks attached to leg infantry should place more value on armor thickness than speed. The infantry was assigned the task of breakthrough, and their attached tanks had less freedom to maneuver. Many modern armies find that being able to fit improved protection against HEAT weapons is worth it, particularly when supporting infantry in close terrain.