Drug cartels are on the decline. Rise of the Warrior Cop is directly correlated with centralization of power and the influence of police unions over politics and public spending.
If you hire mercenaries to beat the crap out of someone, both you and the mercenaries will go to jail.
There are already more private security officers than government police officers in the US today. This isn't new. They have co-existed for many decades.
If often, then there's a market for it.
If not often, then you're intentionally using a red herring to distract from the real discussion.