Personally I think the PLA option is a non-starter at the moment. HK for all its bads is still vitally important to the Chinese economy especially during the current trade war with the USA.
This is why I said "openly."
Economically, Hong Kong is increasingly irrelevant to China - it's only like the fifth largest city by GDP, and there are 10 more catching up quickly. HK is far more important as a symbol, and if I were Xi Jinping headed into a turbulent global economy with flagging growth at home I wouldn't want it to look like I couldn't control my people.
That said, it's a delicate balancing act. Too little response and you seem weak; too harsh and you draw international condemnation and confirm the worst fear of everyone who is counting on China to join the world order rather than try and upend it.
BTW, HK while definitely not the economic powerhouse it once was, it still has a pretty special place as a truly international city for the PRC government, which is something intangible and cannot be expressed in pure GDP number.
Whatever one's opinion of what the protesters are demanding they are breaking the law and rioting (as per Hong Kong law that dates back from British rule), and many of their actions are simply not acceptable. They are really not helping themselves.
The police will respond to that anywhere.
If anything, I think the threats of involving the army are more about (1) deterring supporters and (2) planting the seeds of this idea that this is something they can do, to give themselves the option in the future. I could be wrong, but I certainly hope not.
If the army rolls in, I'll probably seriously reconsider sticking around and might book a one-way out of here, because at that point it might well turn into a war zone.