This is true in theory, but not in practice. Since the selling point of DCS is offering an integrated combat environment, the main development goal should be to expand on that integration (IE modules that have synergy with each other, think A10C and F15C) to create a more valuable product in the end.
Products like the Mustang, while being fantastic modules on their own, simply don't integrate well with the rest of the game, and in the end don't add enough value to the whole package.
Even if you manage to attract a customer solely interested in WWII dogfighting, he'll end up with a very limited game, which dilutes the value of the brand.