Missiles pointing at Taiwan

I have done some significant study of the issue of Chinese missiles pointing at Taiwan, and I cannot understand why China continues to build up more. I'm left inferring that China is worried that behind an impenetrable defense, Taiwan would be likely to declare independence. However, Taiwan's defense has not been impenetrable for awhile, and China has more than enough missiles to at least make a contest of Taiwanese airspace by annihilating much of Taiwan's air defenses.

If China is hoping to actually achieve superiority, though, in order to eventually conquer the island, it is not going to work. This is because Taiwan will respond to all moves by China to increase its threat over Taiwan, such as by buying F-16s as they are currently considering. If China wants Taiwan to buy F-16s, this is the perfect ploy.

Maintaining rough deterrence should be the rational goal of both sides. Sure, Taiwan wants to be independent, and China wants to retake Taiwan, but I can't imagine either considers those goals to worth the costs. With that in mind, why is China needlessly increasing the costs, especially when Taiwan has signaled (through not buying everything America is willing to give it) that it is fine with the level of deterrence China has over it?