The ultimate goal is to isolate China in Trump's trade war. The same goal would be employed if China sends in the troops into Hong Kong. The US is using up all its leverage in Trump's trade war and trying to convince the rest of the world in helping achieve that goal. If that happens, dead Hong Kong protestors everywhere. What are they going to do? Slap sanctions and tariffs to punish China...? China has nothing to lose which is the only thing holding China back from violating everything...
If PAP goes into HK is really that big of a deal against China, the West would have already slapped huge sanctions and tariffs on the so-called Xinjiang "concentration camps". If the PAP really goes into HK, it would have been easy. Because the PAP has a lot of advanced toys, training and new tactics for crowd control and stabilizing (维稳). Since guns are not legal for the general civilian in HK, a HK under the control of PAP would be a lot easier to establish public order than the US during the occupy movement. This would have meant a quick and bloodless effort that quickly put down the riots. The western media will simply not have enough material to chew on, because what they really need is an ongoing state of unrest, in order to fuel their MSM rhetoric.
Especially when now the rioters has declared to the world they will sacrifice their lives to fight for what they want, they think they are real revolutionaries and martyrs. It would be the opportuned moment for the PAP to come in and show the world their bluff. Because they are NOT revolutionaries and martyrs. They are cowards. Had they have a ounce of revolutionary zeal. They would have at least bomb and the PLA Hong Kong garrison. Yet they act like obedient little rabbits when it is a real PLA soldier staring them down. All peaceful and orderly facing the PLA soldiers, while beating down on unarmed HK civilians, Mandarin-speaking tourists and HK police.
This is what convinced me that Beijing will not send in the PAP, as long as things go on like this in HK.