They have to break your monopoly first. The moment someone breaks your monopoly, everyone are free to stream in.. this means that you really need 90%+ compete chance in your CoT's to be able to maintain a perfect monopoly, and this is only really achievable if you are full mercantilism and own all the provinces in the CoT.
So the 96% complete chance in the COT screen is not accurate? When I open the COT ledger screen it says that I have a 96% chance. But I'm not sure if that is my chance of unseating one of them or my chance of defending the monopoly. I have 6 placed in the COT so I have the monoploy. And yes I'm not at -5 on the sliders, I think I was only at -1 as I was moving to free trade.