While I do agree with the sentiment he very likely could rebound, I guess my though process is what does opting out really get him? He'll be 28/29 which is still a good age range but unless he shows more power is anyone seriously going to pay him more than the ~$21 mil he'd have left AAV? There's only like 28 people in the majors right now making $21 mil or more. Of those that are OF you're talking about Ellsbury, Kemp, Werth, Justin Upton and Cespedes. Trout will quickly be there as well but most of those deals haven't really played out like you'd want. Supposedly he took less money to come to the cubs but I'm not entirely sure how much less that truly is. And obviously there's the quality of life stuff. Players(outside of Montero and apparently Hammel) seem to love playing for Maddon. Heyward is a big character guy on a team build upon high character players and a team that should be competitive. If you're someone who works as hard as he does you're going to want to win. So while you might be able to get more out of a fringe contender who needs you more, I think you'd prefer to be on a winner.
In Heyward's case, I think the opt outs are more of a poison pill which is to say I think they are there to give you a reason to renegotiate. Assuming he's happy and assuming the cubs are happy with him, he can basically sit down and say he will give up the opt outs if they tack on say an additional 3 or 4 years putting him in the 36-37 year age range at the end of the deal. And in the end it's sort of win win for both sides. Heyward gets some control in case things didn't work out with the cubs. The cubs didn't have to commit beyond his 33 year old season initially and it also bought them time pre-renegotiation of their TV deal.