I don't really see how Heyward's value is tied to his legs. He's an excellent defender, and even as he expectedly declines in that aspect he should still be comfortably above average (Alex Gordon is much older and still elite). He's a great athlete and seems to keep himself in excellent shape. I also believe that there's also untapped monster potential in that bat, but even if he doesn't break out he's still likely a consistent 110-120 wRC+ bat.