I've played out some variations of this on the game board. As weak as the USSR is in 1939 Hitler is in real trouble if Stalin joins the war and cuts off his resources. A effort to focus on conquoring Russia leaves France unmolested and free to rearm and retrain its army and air forces.

With French air fields just 100 km from the Ruhr the Allies dont have to wait untill 1944 for long range escourt fighters. They can begain pounding German industry to rubble in 1941.

A more ineresting variant is to switch the roles of the USSR and France. A fearfull French government comes to agreement with Germany, hoping for time to rebuild its military. Desperate the Brits and the USSR form an alliance and try to save Poland.