I think Heyward has some - if not the lions share - of the culpability for his decline. He was/is a tinkerer, forever trying new stances and planes and positions in the box...he never did have much loft, but rather than hitting the dog**** out of the ball at an average launch angle of 10°, he started going out and around everything trying to lift it and has never gotten back to where he was. Remember the year ('13?) he started his stance holding the bat pointing toward the umpire? Awkward as hell.
I was, at one point many moons ago, a pro bowler...but before I got there I became convinced I needed to square myself at the foul line in order to maximize my accuracy and power. After two years of trying to accomplish this and being woefully off-balance, I went back to my natural angle 15° off center - which you'd never teach - but because of some unique stuff in my armswing it worked for me. And I got very good.
I wish Heyward would look at his old approach, like, eight years ago, and try and get back where he was. His plane was off, but he got to most everything and hit it all hard.