I generally don't subscribe to the Great Man theory, and there were other Soviet competent commanders, but in the case of Leningrad, I think it would have been lost if not for Zhukov and that it would have been a devastating blow to the Soviets. Voroshilov's orders would have led to the capture of the city and the destruction of the armies of the Leningrad Front along with it. Perhaps another commander could have set things straight in Moscow in 1941, as their advantages at Moscow were decisive in terms of fresh well trained troops going up against exhausted Germans at the end of their logistics train with non functioning vehicles, but in the case of Leningrad, I just don't see it.
If Army Group North had a free hand in 1942 to strike east, Murmansk and possibly Arkhangelsk could have been cut off from Moscow, and the Red Army may have starved as a result, because without American imports, they not only wouldn't have the food to support the war effort , but they would not have been able to motorize and later on, mechanize, the Red Army. Without that motor pool from L-L, there is no Bagration, and the Red Army's offensive abilities are severely limited, and there are still numerous issues with communication and transport, while the Germans do not lose their decisive advantage of maneuver warfare.
The fall of Moscow would, by the way, doom the Russian war effort, if the Germans were able to hold it, not that the capital city's loss would lead to instant surrender but rather because Moscow was such a crucial logistical and strategic rail hub that its loss would cripple the ability of the Russians to get reserves up and down the front and would separate their armies from each other, allowing for the possibility of massive pockets forming once more.
