fing0lfin, I completely agree with your argument about the eastern front.
The premise of this argument is that Hitler had made no plans to invade the Soviet Union, and not that he planned the invasion of GB from the beginning.
Anyway, until early 1941, neither Hitler nor the Wehrmacht had committed themselves to Operation Barbarossa. From May 1940 to Winter 1940, the undivided and sustained attention of the Germany was focussed solely on Great Britain. In this period of time, Great Britain was at the low point of its military strength (Dunkirk), and Germany at its height. After that, the gap started to close, especially between the Air Forces, as the Luftwaffe failed to destroy the British plane industry in the Battle of Britain. "It's the economy, stupid", and Germany's head start in terms of aircraft production and pilot training was melting like ice in the sunshine after May 1940. Can you name just one reason why Germany would have won a second (Air) Battle for Britain, under worse conditions?
Another reason why Germany would not have won the war in the west: Hitler would have been forced to deploy most of the Wehrmacht close to the Russian border anyway, because an attack of the Soviet Union would have been possible at any time. Not planning to attack the Soviet Union doesn't mean that you are safe from being attacked by the Soviet Union. So don't try to argue with me that Germany would have just switched to producing solely planes instead of tanks without Operation Barbarossa. Even with a considerably boosted emphasis on aircraft production, the RAF would have been too strong for the Luftwaffe. Furthermore, in our hypothetical scenario the German situation gets even worse after June 1941: if the Soviet Union didn't join WW2, then the substantial number of US planes and resources sent to the SU under the Lend-Lease programm would have been sent to England instead.
1. I never said that.
2. This whole discussion, as every historical what-if question, is Sci-Fi in itself.