Maybe this looks obvious looking back with that 20/20 hindsight, it's even possible that it's true. It's also possible that it's just Monday morning quarterbacking. The Allies opened that third front in 1943 with the Italy invasion, tying down 70 German divisions which left only 33 to defend against the '44 Normandy invasion. The U.S favored a '43 France invasion, Churchill saw Italy as "the soft underbelly of Europe" if which taken would have opened the road to Berlin. Given the devious and utterly amoral natures of both Hitler and Stalin it's entirely possible that without the third and fourth fronts (Italy and France) they could have decided to remake their alliance of convenience with the line somewhere in Poland, leaving all of Western Europe in Nazi hands. We'll never know and making the kind of flat claims that you do about this kind of unknowable seems like an overreach.