And if the Eastern front hadn't existed Germany would have a lot more forces to fight against the channel invasion. So... nobody fought in isolation, the world always was connected, and trade and impact of what others did elsewhere always existed since there as no force field around any two parties to ensure they had a "fair" fight 1:1.
Also see my other comment, about the production issue, Russian achievement is really huge: https://news.ycombinator.com/item?id=29836874
The US supported itself with Lend-Lease. Giving weapons to others ensured they didn't have to deal with a victorious Germany.
Quoting:
> David Glantz, the American military historian known for his books on the Eastern front, concludes:
> Although Soviet accounts have routinely belittled the significance of Lend-Lease in the sustainment of the Soviet war effort, the overall importance of the assistance cannot be understated. Lend-Lease aid did not arrive in sufficient quantities to make the difference between defeat and victory in 1941–1942; that achievement must be attributed solely to the Soviet people and to the iron nerve of Stalin, Zhukov, Shaposhnikov, Vasilevsky, and their subordinates. As the war continued, however, the United States and Great Britain provided many of the implements of war and strategic raw materials necessary for Soviet victory. Without Lend-Lease food, clothing, and raw materials (especially metals), the Soviet economy would have been even more heavily burdened by the war effort. Perhaps most directly, without Lend-Lease trucks, rail engines, and railroad cars, every Soviet offensive would have stalled at an earlier stage, outrunning its logistical tail in a matter of days. In turn, this would have allowed the German commanders to escape at least some encirclements, while forcing the Red Army to prepare and conduct many more deliberate penetration attacks in order to advance the same distance.
> Left to their own devices, Stalin and his commanders might have taken twelve to eighteen months longer to finish off the Wehrmacht; the ultimate result would probably have been the same, except that Soviet soldiers could have waded at France's Atlantic beaches.
From https://en.wikipedia.org/wiki/Lend-Lease
Interestingly this American historian seems to believe Russia would have won anyway - with grave consequences for the West.