Why would the Soviets collapse in a timeline that has them avoiding the horrible experience of WWII?

The U.S.S.R. was modernizing at a breakneck pace before, during, and after WWII. Stalin is still likely running the show until the 50s. The Soviet Union will only be stronger without the ravages of WWII.
I think Hitler would stay in control of Nazi Germany even if it does undergo an economic collapse. The Wehrmacht stuck with Hitler to the bloody end OTL, a simple recession isn't going to be suitible motivation for a coup. Even if a half-hearted attempt like Valkyre happens, my opinion is that it will end with a purge of the officer corps and a strengthening of Hitler's grip on power. I think that Hitler will "miss the timing" and muddle through a economic downturn as Britain, France, Italy and the Soviet Union modernize and get their respective acts together. Nazi Germany will probably still become an economic powerhouse in the late 1940s, and start sponsoring proxy wars in Central Europe.
The United States probably comes out of the depression early 40s, but it won't be as clearly a superpower. Its military will be smaller, and isolationism will still have a following. Japan will probably burn out in China and head home after a few years. Everyone was sponsoring the Chinese, and Japan would have run out of blood and treasure sooner or later. They'll know that taking on the undistracted west is pointless and go quietly. Japan may lag behind the rest of the world and become a second tier militaristic power, akin to somewhere like Argentina.
Eventually nuclear weapons will be invented and the world is frozen into alt-1950ish. America and Russia will be Superpowers, and more evenly matched than OTL. Britain, France and Germany will be Great Powers. Britain might even masquerade as a Superpower for a while without the bankrupting experience of WWII, but eventually the colonies will go their own way. China, Japan, and India might become Great Powers, but it really depends on how the Sino-Japanese war plays out, and on what terms India gains independence.
I'm not sure about a sure war in the West. Without Hitler in charge, the Western democracies might simply try to play the Germans and Soviets off each other until they both collapse.
If there's no invasion of Poland, appeasement will have worked. The idea was to play for time so France and Britain could build themselves up to a level where they could be sure of winning against the Nazis. By alt-1941 it will be suicidal for Germany to try taking on the Allies. Germany had pretty much maxed out on its ability to rearm and 1939, and Britain and France were just gearing up. The Allies will feel much more secure in telling Hitler "no" in any major diplomatic crises. If Hitler does press Poland about something in 1942 or whenever, the Allies will come down on him like a ton of bricks, Soviets be damned.