Why are pitching coaches supposed to magically fix all pitching issues? If you are right, than it was idiocy to pay big bucks for Price and Kimbrell because the pitching coach could have made Buch or Kelly or whoever into a top of the line starter and Tazawa into a great closer.
There is a reason why the average AL team has 12 pitchers to cover just one position (which doesn't go to bat) and 13 to cover the other 8 plus the DH. Pitching is just plain hard to do well. You have to be able to throw reasonably hard, but also have a variety of pitches, and, most important, have incredible accuracy. Mike Mussina of Orioles and Yankees fame once said that, if he started 32 games in a season, he would have all his pitches maybe a quarter (or less) of the games, just one or two good pitches half of the games, and nothing much working in 1/4 of the games. This year we have seen Price struggle, but also be brilliant. Same goes for our top of the line closer, Kimbrel, and he only has to pitch one inning at a time. Wright has excelled in part because the knuckler only has to be in the strike zone, not in the corners, but even he has struggled now and then.
As for Buchholz, I think his big issue is some combination of confidence and control. These days he simply doesn't go after hitters and prefers to nibble but doesn't have the great control to make that work. Same goes for Erod who has shown he can pitch well when his changeup is working and his fastball is hitting the right spots, and the occasional curve or slider is effective. As soon as Erod or Buchholz relies heavily on fastballs and cut fastballs, they are dead, but they are forced to do that when they can't hit their spots with the breaking stuff. This is very analogous to Wright's situation when he can't throw the knuckler for strikes and is forced to throw that 86 mph fast ball which the hitters are sitting on.
I'm not saying pitching coaches aren't important, but am saying that good, consistent pitching is uncommon, which is why it is so expensive.
If pitching coaches were as good as you imply, some of them would be paid a whole lot more than they currently are.