How about nations that lock their stats see a gradual decrease in those saved stats, like you lose happiness in peacemode.
For every day you're locked you lose a certain precentage (0.5, 1, not a huge amount) of your stats. This would continue to decrease until you come out of lock.
This way people can't just stay locked forever and not bother rebuilding their nation and they also don't get a single massive stat decrease, it's a more gradual one.