Very few pitching (and hitting) coaches are actually qualified to make major mechanical changes and I think for most leaving a players core mechanics alone and only making a few tweaks here and there is the way to go.
Of course there is some stuff you can Improve, especially balance, footwork, Stability and so on but with stuff like changing arm slot I wouldn't want 99% of coaches to ever touch that.
There are some guys I would trust with that like for example driveline but with many coaches mechanical changes are simply guesswork based on what they think is right without really understanding the biomechanical reasoning and down stream effects that a change could have.
I'm not saying a PC should never work on mechanics but very few are qualified to make changes to the core of the mechanics and those coaches are better off focusing on stuff like pitch mix, scouting, game planning and so on and only make mechanical adjustments if something got really out of whack.
Completely rebuilding a pitching staff from the ground up could be a recipe for disaster if you are not having big resscources like a biomechanics lab and very qualified coaches.