He has also had the least amount of assists in his career. Not his fault, just a function of plays that came his way and my argument is that another RF could have offered similar defense with better offense and less choking at bat
Don't really see what assists have to do with matters. They are entirely dependent on men being on base of which the cubs are at a near historic pace for. But, saying there's RF who can offer similar defense is just flatly wrong. The only comparable OF to him are almost entirely CF's and there's not even enough quality CF to cover all 30 MLB teams. And that's not just metrics saying that. Scouts will say the same. At the end of the day, Heyward's defense is at a point where "routine" for him is basically 50/50 for most defenders and 50/50 for him is very difficult plays.
Now as to whether or not you could get more value out of a better hitter with slightly worse defense... I mean this year sure you probably could but that's because he's hitting horribly. But the cubs obviously value defense. For example, Baez is hitting .271/.308/.426 to Heyward's .226/.299/.312. You'll get no argument from me that Baez has been the more useful hitter but the point I'm getting at here is it's not exactly like Baez has been an amazing hitter this year. He's a ton of value because he's been great defensively and slightly below league average as a hitter(93 wRC+). Heyward has been great defensively but significantly worse(66 wRC+). But on his career he's 111 wRC+ or around 11% better than your average MLB player.
Regardless, it's largely pointless to have this debate. You have to assume that 2016 Heyward is the hitter he will be going forward which is rather absurd considering his wRC+ is nearly half what his career averages are. No one falls off a cliff that hard. Even like Andruw Jones only fell from 124 wRC+ to 86. All these supposed flaws in Heyward's swing have always been there. It's not like he's suddenly got the yips. It's far more likely that he tinkered with his swing to get more power(cubs talked about him working with Mallee), got that early season wrist injury, and all of that combined to throw his swing out of wack even worse than it was before.
And the thing is it's extremely hard to make in season changes to a swing. Rizzo hit .233/.323/.419 in 2013 and many were down on him. The following year he hit .286/.386/.527. Bryant is arguably one of the 5 best hitters in baseball. It took an offseason for him to fix his bat plane enough to lower his k rate.