this is a good idea, beige only happens on defensive wars, but don't nerf it, cause nations need time to rebuild or they are out with first war lost!
also, you need more incentive for attackers to beige the enemy, 10% loot especially in alliance wars doesn't mean much, and it's better to keep a nation down than to get that money
by increasing the loot, or doing improvement damage to beiged nations, you can reduce the incentive to get one's self beiged. also, increase the incentive of beigeing someone.
fixing both issues. w/o taking the much needed time to replenish armies away from the losing side imo.