Germany's eventual defeat was extremely likely from the beginning of Barbarossa, and the failure to force the USSR to the peace table before the end of 1941 virtually guaranteed it.

Had Germany managed somehow to secure the Iranian oil fields before attacking the Soviets - a vanishingly unlikely proposition - they'd likely have faced the same issue they faced when they occupied Soviet oil fields in the Caucasus. The Soviets sabotaged the operations with such thoroughness that they were not worth salvaging, and the British would have no incentive to leave their equipment in Iran in better shape for the Germans to find.