I see we have different intentions, Deznutz. You want to make national takeovers impossible. And if that is your intention, the your suggestion surely accomplished this.
I, however, see battles for national control as an important aspect of this game, and one I do not wish to destroy. I simply suggest it should be a little harder, and more expensive. As I have said earlier, this suggestion is meant to make belonging to a nation mean more; mainly with "wars and blockades" in mind. It is not a tool to destroy what I consider vital to the game, fighting for national control, but will also make such battles a bit harder. No-one should "own" a nation, the way they own a guild.