As a student of military history and in particular World War II, I can say the Russians would probably have defeated the German military by themselves. The Allied’s extremely damaging bombing campaign reduced Hitler’s manufacturing capacity. Even with the bombing the Nazis continued to increase production until late 1944.
Was D-Day necessary if Russia would have defeated Hitler? Our invasion of Europe wasn’t just to defeat the Nazis but to save Europe from being under the domination of Stalin. Image what political ramifications and situations would not only have existed in the late 1940s but today. Think of a Soviet Union stretching from the Pacific to the English Channel and beyond. They might have crossed the Bosporus and taken Turkey, Iran, and the Middle East.
No, every soldier’s death was much more valuable (poor word) than beating Hitler. Each death helped to shape the future of the world. Never have so few people died to save hundreds of millions of people then and future generations of Europe and elsewhere.
Those people who fought and died thought it was to just beat the Nazis. Few knew how critical it was to be in on the division of Europe. Without them there would have been no Berlin Wall, no Iron Curtain and the fall of the Soviet Union. All of Europe might have been annexed. No United Nations or NATO, just the Soviet Union.
D-Day should be called Liberty or Democracy Day.