I fail to see how Stalingrad could have been captured, since The Soviets were still fighting desperately inside Stalingrad before the Soviets launched Operation Uranus. The Soviets never withdrew from Stalingrad, nor gave it up. If Stalingrad was truly captured, then the Axis forces could have continued on. But they couldn't, because they're still too busy fighting in Stalingrad.

And that's what I mean. People were wondering why the Germans simply did not set up a siege around Stalingrad. Well Leningrad was under siege for what, 3 years? And it was a failure as well.

Stalingrad was in a highly strategic location. The Germans could not simply leave it to the Soviet's hands, even if it was in rubble. It would expose their rear and will be a logistical nightmare. The one thing that Stalingrad gave to the Red Army was time. Time to mass forces, and weaken the Germans enough for a counter attack.