One season doesn't mean a pitcher is proven. They get injured left & right and fall of quickly.
Would you of rather he traded for an old aging vet?
Pineda first half and Pineda 2nd half are two different pitchers mind you. He started great and finished bad. He's far from a finished product. He still needs to show how he was in the first half of the year. He also only pitched 170 innings last year. When his innings goes up what's going to happen? What about when he hit's 200 innings ?
Pineda had a great first half but his second half leaves much to be desired.