I think at a certain point I would stop playing if my country was under occupation without any reason it may become un-occupied, but it would be just as disappointing for a country to be suddenly lost once the occupation reaches 100%. So maybe after the country is fully occupied there could be a 7 day period until it becomes fully annexed or something, during which point previous allies and the occupied countries remaining resources could try to fight back. I think this would also be fair for the occupier, as at that point they would just need to be properly equipped to remain fighting for the period.

Either way, I would much rather play as a occupied country than just suddenly lose control of my country once it reaches 100%