A 24 year old B+ prospect for a Cy Young winner signed at 10 million less than he's worth for next year is not a good trade to make. Especially when he doesn't play the position you want him to, has a high strikeout rate, lacks speed and is unproven on the major league level.
BUST written in big bold letters here.
You need a A prospect, can't miss guy to justify the deal. 20 game winners are hard to find.
From the Mets standpoint however, Dickey must be traded because they can't win with him and they can't win without him so they need to build for the future and get a top shelf prospect or resign him and then trade him when you can OR trade another pitcher.