Hello Radoje,
Thanks for your suggestion. It's a quite interesting topic indeed.
Leaving role-playing aside, are you trying to "fix" any issue appearing when we take in-game mechanics/politics under consideration?
In my opinion, this new change would just make it more difficult for the players to wage war. I believe though that the opposite should happen.
Most importantly though, knowing how Alex codes, if we go with your plan, I can already see all of the nations' cities having negative population.