Khome,
The game already allows us to use isolated and protectionist Economics. By using State and National Corps, we can create relatively closed and self sustaining national economies.
The key difference is that such economies are less productive, uncompetitive, and inefficient. This is represented by State and National Corps being less profitable, and having tighter upgrade limits.
To have players build closed national economies, while also getting the CEO productivity bonus for open competitive economies, would be an exploit.
Frankly, the fact that Players get bonuses for having Corps "privately" owned and controlled Corps, while it's actually the same president owning and controlling those Corps, is already extremely generous by the GMs. Typically CEO's are limited to 6 corps per country. They have already allowed players to go way over that to 20 in countries they control. Expanding that even more, would just make a joke of the "private" corp system.
Signed President of DanNation on LU