Unless 2018 JD does what 2017 JD did. I think there's too much risk not trading him. He'll be 32 next year, there's a chance this is the new JD in terms of production. Teams will still have him next year, and hope he has a strong finish. You still might have a team trying to gamble this year knowing they have another year on him.
A> JD stays the same = you get less next year
B> JD gets injured = you get nothing
C> JD performs worse = you get even less
D> He performs better = less team control, teams still leary of 2017, he turns 32
The only reason you keep JD is not to get more trade value next year. It's to sell tickets, and give fans the false hope this will be a competitive team next year (when deep down we know it won't be possible unless Rogers goes to 200m and even then, it assumes we sign all the good FAs and everyone else exceeds expectations). Of course, Boston finished last then won the WS the following year, so PHI/CIN/TO could all win the WS next year lol.