Purely thinking out loud, Taiwan doesn't need submarines to deny use of the Taiwan Strait to PRC's surface fleet. Minefields can do the job over a much wider ar

Purely thinking out loud, Taiwan doesn't need submarines to deny use of the Taiwan Strait to PRC's surface fleet. Minefields can do the job over a much wider area and much more cheaply. Modern mines can sit passively for extended periods of time and the expensive ones effectively are torpedo launchers. They also have a stock of land-launched (and truck-launched) anti-ship missiles, the range of which exceeds the width of the Strait. The "only" issue there comes in lighting up the incoming PRC vessels to direct the missiles at them, but that's a different problem of detection.