"The real effect of D-Day was not to defeat the Germans, but to give the West a claim on the continent and stop the Soviets in East Europe."
You're saying this with the benefit of hindsight. Operation Overlord was intended to open a second front against a viable enemy. Something that Stalin and Molotov demanded since Germany had declared war on the US. The statement "the Soviets were already polishing off the bulk of the German Army in the east" is a boast propagated by Stalin himself to hide that fact that he froze when the Germans poured into the USSR.
The real reason why there were so many Russian KIA was because Stalin purged the military prior to the war that left the army leaderless. His "not a step back" doctrine that sent waves of untrained conscripts into a buzzsaw probably had something to do with it, too.
If we knew that the Germans were undermanned, underequipped, undertrained, etc., then why did we and our Allies go to lengths to dupe them (see: Operation Fortitude), subject our Army Air Corps and RAF crews to virtual suicide missions into the heart of Germany (see: Strategic Bombing Campaign), and take a gamble at an improbable operation (see: Operation Overlord) that could've collapsed our ENTIRE effort in the European theater?
Sure, some of the German units were reconstituted out of misfits and POWs, but most of them were battle-tested, crack units led by fanatics.
Do not undervalue the efforts of our WWII veterans.
True, hindsight is 20-20. And it does not denigrate the efforts of the brave American, British, Canadian, Australian, etc. soldiers in OVERLORD to say that they saved Europe from totalitarian domination. It is just that, in hindsight, they didn't save it from the totalitarians they thought they were fighting. Stalin may have asked for help, but D-Day coincided with the "Destruction of Army Group Center" campaign that gutted the German army clinging to Ukraine and opened the way into Poland and, eventually, Berlin. My argument is that D-day didn't so much get the Soviets to Berlin as much as it stopped them from going any further.
"some of the German units were reconstituted out of misfits and POWs, but most of them were battle-tested, crack units led by fanatics"
There were some very well-led, crack troops facing the allies. Of course, one side's fanatic is the others hero. Put Patton's speech into the mouth of Rommel, and you have a bloodthirsty Nazi spewing hate. Winners write the history.
This was no boast. The German army was largely decimated in the east. About 65-80% of the German armed forces were in the east, depending on when you do the count. The Germans had already suffered about 3.5 million casualties in the east before the U.S. landed in Normandy. Within 60 days after D-day, the Germans lost another 800,000 casualties on the eastern front. (Look up 'Operation Bagration'.)
In addition, the use of prison troops that charged the Germans with something like one rifle per 10 men inflated the kill ratio. They were expected to pick up weapons from dead Germans to fight with.
The number of Russians that died in WWII is staggering, but a major reason so many of them died was that their goverment considered them disposable and did not equip or train them.