I think you're overestimating how good Demar was that year though. That's not the difference between DD being worth or not worth his contract. That's the difference between Demar being mediocre versus intolerable.
His 2013-2014 was mediocre. It's like JP Arencibia in 2012, versus JP Arencibia in 2013 (before we knew that framing was really a thing). In 2012, he put up 1.2 fWAR, an 87 wRC+, very mediocre, and there was a small subset of the fans who really didn't like him but a large subset who liked him because he was flashy - he hit DINGERS. He performed similarly the year before. In 2012 and 2013, Demar posted eFG's (effective field goals, like wOBA almost) around .46; mediocre, but passable. Fans loved him, though, because he was flashy; dunk contest, and s***.
In 2013, JP Arencibia posted a 56 wRC+ and everyone hated him. In 2014, Demar posted a .426 eFG. Now, the drop off isn't as bad; Demar isn't quite JP Arencibia. But the parallel is there. He's played very poorly, but looked good out of flash.
Demar's all around skill set is mediocre. He doesn't have good ball handling, he doesn't have good passing and he can't shoot. He's not a defender. Throughout his career, he's used that skillset to varying degrees. His 2015 bad, his 2014 mediocre. However, that 2014 isn't really good enough either; while a coaching change makes him more of a 2014 guy than a 2015 guy, that 2014 guy isn't someone you want to pay a lot of money to. It's just not a good skill set, through and through.