I really don't think it's a bad trade. Pretty even trade, imo. Saved about 10 mil for next season. We've got Miller, who still has a chance to be a top of the rotation starter, for 4 years. The prospect has good stuff but just needs to learn how to pitch.
I personally did not like the idea of paying Heyward close to 20 million per year. So I like the idea to trade him and get something back. And I think they got fair value for him.
This.