Dubnyk was terrible his last year here, and it was all on him (well, and the goalie coaches). His technique was atrocious, and what made him better again was going to the Coyotes, and having an actual goalie coach (Burke) completely overhaul his game to be where it was
Multiple reasons for his bad season. He had a kid, so distractions at home. Equipment changes. The Eakins swarm that allowed constant golden chances against in the slot. MacT making every opportunity to trade him and making everyone know he didn't think Dubs could be a #1 goalie in the media in the 2013 off-season. Bad goalie coaching, although it worked for him well enough in previous years to put up good stats.
What a proper GM would have done with Dubs, a guy we developed and invested into for years, would first be to not talk crap about him all off-season and talk about all your attempts to replace him. Next if he is having a bad year make a goalie coach change, and have a little patience with him and don't jump to blame him for all the teams problems (to protect your terrible HC). No legit GM would have ever hired Eakins as HC, so I won't bother going into how Eakins should have been fired less than a month into the 13/14 season when it was obvious to the entire hockey world he was in over his head.
Lots of players have bad years (look at Mike Smith this year who is also coached by Burke, the guy that supposedly recreated Dubs from a broken shell of a man to having equal performance to how he was in 12/13 with the Oilers and Chabot). Doesn't mean you throw them away like trash. Especially guys you developed for years, who have shown they can be good at their position as recently as the previous season, and that you can't easily replace. But, MacT made up his mind even before the 13/14 season that he just didn't like Dubs (like he did with Petry before this season) and he turned into a very convenient guy for MacT to blame for the failed 13/14 season to buy Eakins and himself more time.