I don't think the Soviets can completely cut off AGA. It's not enough to reach the Sea of Azov at the mouth of the Don; AGA can retreat west into the Taman Peninsula, then across Kerch Strait into Crimea. For the Soviets to cut that route, they have to advance another 250 km, which is about half as far as the Germans need to retreat from their furthest positions in Ciscaucasia. And retreating through friendly territory toward one's base of supply is easier than advancing against resistance and away from the base of supply.