And, quite frankly, and the focus of these comments, an act by the PRC to place active S-400 batteries on the Chinese coast to be able to control the air space over, say, Taipei would be one of the surest ways for the PRC to ruin the good progress that is being made towards that eventual goal.
Your point is a bit unclear. SAM systems do not control air space . You could use radars from SAM system to monitor air space (in this case with acquisition radars like 96L6E , 91N6E etc .. ) and you could use missiles to shoot at targets in range . Unlike fighters , you could not fly to a target , inspect it , order it to land or leave . Theoretically , air control could order aircraft to turn back or be shoot at with SAMs, but this is not done this way .
China already has radars monitoring Taiwanese airspace , and this is not a secret . They could employ S-400 radars for this if they want , but I doubt they would do this constantly and accumulate running hours of the system . From Taiwanese point of view, they are used to be being "painted" by Chinese systems . One exception would be irradiation by fire-control (engagement ) radar , readying itself to launch SARH missile, but I doubt Chinese would do such a thing unless they really want to start shooting war .
Strategic implication for Taiwan is not where would China place its S-400 batteries in peacetime . Strategic and worrying implication for Taiwan is newly acquired ability of China to place those batteries across Taiwan in relatively short time if situation deteriorate . S-400 spoils in large degree Taiwanese Western doctrine of relaying heavily on air power, air resupply, air surveillance etc ... Therefore, they are slowly being funneled towards rejecting option of trying to unilaterally declare independence and then resisting with arms . As Chinese military might grows, more and more Taiwan would be put in position where they need to negotiate with mainland .
