Because they will be more directly accountable to their "clients," for one. I cannot fire a "real" cop whose behavior I do not approve of.

You seem to be presuming that you will be the one these mercenaries are working for. When mercenaries I hired beat the crap out of you, how will you fire them?

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.

What are you going to do, hire your own mercenaries to arrest me and mine? Or do you now recognize the value in real police?

I don't think the author ever claimed there's no value in real police. It actually, ironically, matches the recent left-wing demands to defund. Let the police with their armored cars and helicopters handle real crime, like when your mercenaries beat up my mercenaries. Let the mercenaries handle the small stuff :)