No doubt the USSR was key to winning WWII. But your analysis errs on the other side. The US was also instrumental in winning that war- for starters just study Lend Lease.
Now let's look at the direct participation of US. England was virtually exhausted post 1940 Battle of Britain. Churchill was desperate to get the US into the war, turn England into a floating base for the allies. Then, when you study the US combat role in North Africa and post Normandy (# of combatants, materials etc) it's hard to claim that the allies would have won that war without US help.
I hypothesise that without US intervention the Normandy landings would not have happened in 1944, maybe not for the next 2/3 years (if at all). Germany would then have fought a single front war against USSR, and how that would have ended is anyone's guess. Perhaps a stalemate. But this is conjecture. What's not conjecture is the vital importance of US in WWII.