Many historians believe that the Soviets would have crushed Germany even without a Western Front in the picture. The Russians were incredibly tenacious, they suffered millions of casualties, yet continued to move forward. Hitler's gravest mistake was to attack Russia. The German Army wasn't even fully mechanized when Barbarossa began. Neither was the Soviet Union. But their Homeland was being invaded. It gave millions the incentive they needed to repel the Germans, at all costs. Even without our help. I believe the Russian's, underestimated by Hitler, would have eventually prevailed.
The historians you cite always forget about the Starvation Winter of 1943. The Russians would have been seriously hampered but for the fact the USA was making massive food shipments to them.