The Russians weren’t written off…the US supported the USSR right to the end.
Yes, the Russians were underestimated, not by the Allies, but by the Nazi regime and the Axis.
The Russians were not humiliated in the least. They took terrific losses at the beginning of the war(surprise attack by an “ally” who signed a non-aggression treaty), but absorbed those losses, fell back, regrouped, and drove the Nazi’s all the way back to the heart of Germany, killing and capturing millions of the enemy, and ultimately humiliating the Germans.
Don't forget Stalin scorch earthing his own country in order to stop the Nazis. . .it only cost a few thousand plus lives and a famine during the worst of a Winter which killed even more Russian civilians. . .but hey, at least he was able to make the Nazi just as miserable!