Well, the Chinese don't want Taiwan to become another economic powerhouse like Japan at their doorstep, nor do they want a militarized Japan, nor do they want it to become harder for them to sell their slave labor made goods all over the world.
Reigning in Kim Dung Poop and backing off their aggressive stance in the S. China sea seems like a good bargain.
Japan is an economic powerhouse, though in perpetual recession with an aged and aging population, and Taiwan is already no slouch. Do they want a militarized Japan? No, but that ship has pretty much sailed, and it's not like a Japan with the militarized U.S. backing them has stopped them from largely doing what they want in the South China Sea. No reason to suspect Japan will be able to either, and it's not like Taiwan and Japan are allies on the issue. They're unfriendly about it towards each other. And it's not a good bargain to China, obviously, or they would have already done it. The Kims have been a problem for China for a long time now, but that's preferable to China than destabilizing the regime and dealing with the disaster that will inevitably follow.