The heroine is not only a substitute, but also the real person whose identity has been replaced. Bai Yueguang, whom the male protagonist thought was dead, was actually an imposter, and the female protagonist was the real Bai Yueguang. This reversal is cruel enough - the male protagonist is torturing the lover he couldn't find, and the "relics" he carefully preserved are all fakes of impostors. When the DNA report shattered all his knowledge, the feeling of collapse should be very impactful.
