You make it seem as if the Soviets had unlimited manpower. By 1943, the Red Army was having major manpower shortages and relied on tactics to keep from losing to many soldiers. Also, I believe Moscow was a transport hub and all land west would be hard to fight for without it. And most Soviets live in the west. They did not have unlimited manpower. I believe the Germans had about 40% of the Soviet population by 1942. That means about 150 million still in Soviet Union. With most under German influence, the Soviet numbers get lower. Time is not on the Sovit side as much as people believe it was.