There's also a problem with the idea of a "defensible line" in Russia. The original grand strategy behind Barbarossa envisioned a defensive line stretching from Archangel in the Artic down to Azerbaijan in the Caucusus..the so-called "A-A" line. This assumed that the major Soviet cities..from Murmansk, to Leningrad, Moscow, down to Stalingrad..would be conquered. They envisioned large armored forces, backed by the Luftwaffe..patrolling the vast Russian interior. I've read a synopsis of the plan..it's fascinating to see the German mind at work..but it wasn't realistic.
Slightly off topic, but for those who enjoy alternative history, I stumbled across a fascinated novel.."Dominion" by C.J. Sansom. It's set in England in 1952, 10 years after Chamberlin agreed to an armistice with Hitler. The Fascists run the government, Oswald Mosley's a big deal...and Churchill's the leader of the rebellion. A good read..worth looking for.
Probably, IMO, the first, and actually the best alternative history was by CS Forester..best known for the Horatio Hornblower saga. He wrote several volumes of short stories exploring what if Hitler had succeeded in invading England
A tome I read about Stalingrad said that Stalin, in a weak moment within the first 60 days of the invasion, was willing to concede these in a peace treaty if Hitler would stop the offensive. It was the Bulgarian Ambassador who assured Stalin, that, ultimately, Russia would prevail, if they just had the willpower, in a war of attrition.