The mental health professionals that replace certain officers will technically be cops also. Just not ones trained to turn violent because you don't immediately get your way.
What happens where the subject turns violent? Does this mental health professional, riding with the police, ask the subject how he feels about being high on crystal meth, being in possession of a dealer amount of crystal meth, having an illegal firearm in his possession, and already having two arrests for dealing and about to be arrested?
A lot of mental health professionals believe themselves to be quite competent at working with people in crisis, or people who are psychotic, but what they forget, is that the overwhelming majority of the patients
they have seen are the ones who held it together long enough to go look for help. The rest, who didn't, who don't want any help, and who are spazzing out in the middle of a busy street and stabbing anyone who comes near them, are a bit outside of their experience, and they don't have a freaking clue how to approach that person.
Cops, ambulance drivers, and ER personnel are all far more familiar with folks who are in this state than the shrinks are, because the shrinks don't even see them until we all got them settled down enough to stop trying to bite everyone. And that often involves using force, putting them in restraints, and sometimes injecting them with some "calm-your-ass-down" juice.