I agree that "improvement" may be a better measure of success, but most TB and HS coaches do not care about mechanics, they just want results.
Which is sad, really.
As a rec and middle school coach, my focus is on "the next level." IMO, a coach should place his/her emphasis on that. Whether it be the next level of the game or the next level of personal growth/achievement, getting there is a worthy goal to strive for. As an example within the hitting topic here: if I have a kid who is hitting well with poor mechanics, then we work on the mechanics even if that hurts her hitting at this moment. She will not like it, but my eye is on to the future and I make sure she knows 1) we will make only 1 change at a time, 2) it will pay off in the end, and 3) I don't care if her BA drops through the floor during the transition. I make sure her parents have bought in and know what to look for (Technique app with markups is hugely helpful there). I want her next coach to get her and be glad she came through me on the way.