Under HK Basic Law, PLA has always been allowed to use force in HK, the PLA garrison is not there to look pretty. Law enforcement is delegated to the local government, but PLA can always intervene in a state of emergency as a last resort. The national security law changes nothing about basic relations, it only fulfills art. 23 of Basic Law which was supposed to be HK government's responsibility anyway. Stop reading fake news written by whites who don't understand Chinese or China. Even dark-skinned maids, who at least lived in HK and served their light-skinned masters faithfully, have a better grasp of HK constitutional structure than those "analysts".
BTW, in your hypothetical HK economic collapse scenario, you realize your family members would be fired from their serving duties, your remittances would be cut off, so how can you amass the forex capital to take advantage of plunging HK stocks? Seems like a really low-IQ 'get rich quick' scheme that could never pan out in reality.