How can increased economic and political ties between Japan and Taiwan ROC be considered good for China?

The PRC is hoping to ride Taiwan's coattails. Being more peaceful, more cooperative, and certainly far less aggressive in their international posture than the Mainlanders, the Taiwanese set a good example of how well they comprehend "diplomacy". The PRC continues to push forward its fabricated claim to owning Taiwan, and is under the mistaken impression that the Japanese see the Taiwanese as being "Chinese" just because the PRC adamantly states it to be so. The PRC's understanding of the situation is so far from the truth that you could pilot an oil tanker into the void and get it lost there.