Warm bodies are a lot more scarce than they used to be. In the 1930s, the Soviet economy was massively more forgiving than today. A LOT of them were basically little better than subsistence farmers, and agriculture was rapidly industrializing. This made for an amazing opportunity to extract a LOT of manpower from the population. And much of that force was "frontoviki", which is basically the lightest of light infantry, a guy with a rifle, helmet and some spare ammo. That's not how you wins wars today. Today, Russia has a massively aging population, and many of the younger people who could get out, got out.

Material is a lot more scarce than it used to be. The Soviet Union included Ukraine, and that had a LOT of factories, which were moved to the east when Germany invaded them. But back in 1941, you could make a tank basically with a rivet gun and a couple of wrenches. They were building airplanes quite literally in farmers barns. None of that applies anymore, you need massively more technology, and throwing more people at it won't get you, for example, more jet engines, radar absorbent materials or advanced night vision sights.

The lend-lease is going to the other side this time. People hugely underestimate just what an insane amount of material came from the other allies. The majority of high octane fuel and almost all airplane fuel was American. Almost every locomotive running in 1945 was American. Every other bullet and bomb fired was American. Every third truck was American, and many of Soviet ones used american or British engines. Basically every radar and sonar set was British. They're still digging up British Hurricane fighters in Ukraine that the soviets buried rather than giving back. The allies sent Russia almost half a kilo of food per Soviet soldier per day.