@Ironman's idea reminded me that it was always a debate if its good or not that countries can be deleted. To satisfy both sides, we could add a huge defense bonus (it can be calculated from the average of the last 10 battles lets say) to the capitol of every country. Then if it's under occupation, it would revolt automatically and randomly in given time periods. This way it's still possible to delete a country, but it will cost huge resources for any nation to keep it deleted. How hard, we can simply adjust.
Edited by Zoltan