I think the bigger issue with the defense (at least at a position like CF), is that in general, speed declines much faster, and at a younger age. If I remember right, there's a big drop off around 23 to 24, and another in the 27 to 29 range. This is likely due to the addition of muscle, which is typical for players in these age ranges. There ARE players who see very little decline in their legs, but not many. My fear with someone like Heyward, would be that a reasonable loss in his speed probably turns him into a 2.5 to 3 WAR player. Still plenty valuable, but not the 5+ WAR guy he is right now.
Trout is a perfect example of the speed decrease (although his bat is so damned good, it more than makes up for it).