The problem with the siege theory is the fact that the Germans already had one city (Leningrad) under a tight siege, and that was an utter failure as well. The Germans were simply too stretched out. Even if Hitler decided not to be a egotistical fool and demand for his generals to capture the city bearing Stalin's name for prestige, and bypass Stalingrad altogether and go to the Caucasus, the Soviets would have simply wore the Germans down until they could counter attack with a massive force.