You still ignore the fact that the war had been raging on for a while before that.The Germans faith was sealed at Stalingrad, around 300,000 casualties many of them their most experienced troops. Follow that with the battles of Moscow and Kursk. If D-Day landed failed it just would of meant more Soviet losses but they still would of defeated the Nazis
The allies did not appear out of the blue on D-Day. Russia was not fighting the Germans alone.