The kingdom got a whole army of new slaves suddenly. They would also empty the church coffers, logically, and take all the gold and silver, no matter what form it might be in. Some of the loot should go to Naofumi for fairness's sake. Royals have always been looking for excuses to steal other organisations' wealth, like happened during the reformation, for example.
Actually, he was powering up his spell. It was highly convenient for him that the heroes kept talking, doing nothing else.