That line of progression makes sense since you have some overlapping of aircraft that you can either straight-up reuse for the next addition or slightly tweak to get the period appropriate model. The problem is that the Eastern front will probably be a low point for sales outside of Russia, and the Med/N. Africa isn't the most popular theater either. Cheaper to make the game and sequels this way, but limited returns.

I'd like to see what their marketing dept had to say about that path and the reasoning behind it. (assuming it happens in that order)