Fundamental flaws?
You mean things like “They can’t drive through buildings without knocking the entire crew”?
Nah, light tanks don't have the structural strength and horsepower to carry large enough cannons. If a light tank shoots a WW2 heavy tank, or a modern tank, nothing happens. Might as well be blowing kisses at it.
No, no no no, and no. The problem is that they were designed for a role better filled for other things (scouting and anti-infantry work), but then deployed in antitank roles, to which they were unsuited, NOT because they couldn't carry a big enough gun, but because they simply DIDN'T carry a big enough gun, because it wasn't necessary for their role. Rather like the Shermans or Panzer IVs not packing really good AT armament, because something ELSE was supposed to fulfill the role of killing the nastier enemy tanks.
If you wanted to, you could weld a 128mm flak gun (see: Maus tank, Jagdtiger for examples of the gun) to the top of a light tank. It might flip the thing, but it probably wouldn't rip it in half or anything so dramatic. However, you just compromised its ability to scout, move extra-super-fast, or be dropped from a plane, the three things light tanks could do that a medium couldn't usually do.
Yeah, and a 128mm flak gun wouldn't do anything at all to a modern tank. Or most WW2 heavy tanks. Hence my point.
If it was trivial to fit smaller tank chassis with larger guns, why did so many of them get modified into awkward, inferior anti-tank designs with immobile cannons? I mean heck, why are various modern nations strapping ATGMs to the sides of their APCs when they could just put a MBT cannon on it and have a weapon that, you know, keeps the explosive payload INSIDE the armor. And can fire more than once. And turn. And be worth anything at all in hilly terrain.
The Sherman was not a scout or an IFV. It was our primary tank in WW2. The same thing happened in WW2 as in WW1, which is we entered the war late and lacked the practical knowledge that the European powers had. In this case we tested out the Sherman in Africa and found that it was superior to the German tanks, not realizing that the Germans were unwilling/unable to deploy their heavier tank variants there.
It worked out anyway tho. The Germans were critically low on fuel because of the eastern front, which is also were a lot of their tanks were. Meanwhile we had so much production we could drown the Germans in tanks and the Japanese in boats. Plus we made reliable stuff that we could repair. To put it into perspective, by the Battle of the Bulge the Germans were so low on fuel (and they were so far past the point of practical tank size) that they estimated that their armor would run out of fuel less than halfway to the objective and they would have to steal ours. If we were fighting the Germans in video game terms (equal sized armies), hell yeah the Sherman would have been totally insufficient. But the thing you have to understand is, WW2 tanks just broke. I don't know why, and it depends on the faction and the year, but the reliability was awful. Without a constant influx of spare parts a WW2 tank unit would basically defeat itself. I don't think we even destroyed the enemy heavy tanks in the battle of the bulge, IIRC they broke down or surrendered. It was the same story on the Eastern Front; the Russian heavy tanks were infamously durable to the point where they would have their treads destroyed and become an immobile turret until they either ran out of ammo, a tank got behind them, or a German soldier pried the hatch open. And likewise the Russian tanks were, even by WW2 standards, horrifyingly unreliable to the point where even when the war was at its worst and the front wasn't THAT far from the factories, they would still break down on the way there.
Re: reactive armor: reactive armor blocks anything once. Great against one big shell, useless against a lot of little shells. And modern targeting and ammo aren't super special. As far as I know the same targeting technology that's in our tanks now was in our ships back then, its just that it was the size of a house. Our tanks can't see through walls, they can see in the dark or fog sure but again, that tech is cold war era at worst. As for ammo, all mainline battle tanks in every era come standard with whatever the best AT ammo is. Its not like tank commands are going "shit that tank is firing HEAT at us!" Its just kind of assumed, they'd probably laugh if they realized someone was firing HE/anti-infantry shells at them. I'm sure that our shells have gotten better over time but armor has gotten better with it. MBT exist because they either out-armor or out-range 99% of the weapons on the modern battlefield, and what's left is difficult to use things like rocket pods and ATGMs (that are generally off limits to anyone fighting a guerilla-style war). If something like a Stryker could mount a cannon to take out something like an Abrams, we would not use Abrams tanks.