[lyubalesya]

Without immigration, they will be forced to fix their broken system that has resulted in far-below-replacement fertility. With immigration, they can persist with this soul-crushing work schedule, while their native population dwindles.

There is not some fixed amount of work that needs to be done (in Korea specifically, that cannot be outsourced), and Korea simply doesn't have enough people to do it. In fact, if that were the case, then workers would have the advantage in negotiations, and could paradoxically bargain for better working conditions.