Because if China decided to use force it is not only exercises, it is not bluffing, it is not intimidation. It is a decision to use force to damage Taiwan, and make Taiwan to come to the negotiating table with China.
The number of missiles has been steadily increased. In another 2 to 3 years, Beijing will deploy another 500 to 600 missiles. So definitely Taiwan is under tremendous threat and pressure,
I don't think that there is going to be serious impact to trade between Taiwan and China. It's not in China's best interest to cut off trade.