Kostin needed a fresh start. He has squandered multiple opportunities to claim a 4th line spot on this roster. Last season the door was wide open with injuries, and he got passed up.
He has good enough physical tools for the NHL. But he is a one-man show and doesn't seem to know how to use his linemates to develop a play. I think the desire and effort are there, but the hockey sense just is not.
He got some very nice comments from coaches in the KHL and appeared to be a contributing member of the Gagarin Cup winners a couple years ago, but we never saw that mean anything in North America.
I suspect neither guy in this trade will amount to anything at the NHL level, but they're both at the stage of their careers where developmental potential is getting pretty thin. A fresh chance in a new organization is the last hurrah. Good luck to Klim!