Excellent article. A lot of marginal MLB ballplayers are superb athletes with so-so-to-miserable mechanics. Bad habits are learned early, reinforced when the special athlete excels over his competitors and then are hard to break due to (however occasional) positive reinforcement, ignorance, stubbornness, ego or stupidity.
Kudos to Vavra, et al., for identifying area of improvement and, especially, to Plouffe for heeding the advice and being able to execute the changes. I hope he can keep it up. MLB pitchers are best at finding and exploiting (new) weaknesses of hitters. They never give up. It is what separates them from the animals.