Who says that it is actually easier to play as the losing side than as the winning side?
Sure, you can conquer the world with Germany, you can conquer the world with pretty much every other nation in the game. People have done it with Lithuania, Albania and maybe even Luxembourg. Being able to win with Germany doesn't mean that it is easier than doing it with the USA, Soviet Union or Great Britain. If you are somewhat good at the game, none of the big nations are particularly hard to win with.
In the end, what needs to be done is what is the best for gameplay. If the developers can pull it off to have every nation with their historic military strength while ending up with a historical result pretty much every time, then yes, the historical setup would be great. If the AI can't pull it off though, changes need to be made. Having France or Poland at their historic strength is less important than having WW2 play out somewhat historically. You don't want to end up with players complaining because they spend a lot of effort in a US-game, only to have Germany fail beating France 50% of the time, thus making the whole game more or less irrelevant or broken.
Making a nation a bit weaker is a whole lot easier done than forcing the AI into behaving stupidly (as in, making a better AI act stupid).
Read my point about not all AIs behaving the same. The France AI would be slow and disorganised, while the German AI would be far more astute and aggressive. 1 AI for all nations does not, has not and will not work because not all nations acted or reacted the same way - contrast French and German tactics and strategy for a clear example, or the change in Soviet strategy and tactics throughout the war, going from being encircled and demolished by Germany to successfully defeating them again and again. Without changes in AI behaviour, this cannot be represented. With changes, a historical WW2 is completely possible with all nations at historical strength.