I don't think fit or becoming a better 2-way team are the big reasons why a Kessel trade may make some sense. The biggest thing is that you don't want to be the team holding the ball when the music stops, you don't want to have Kessel after he falls off offensively because you're stuck with him at that point. You're going to have the next Alex Semin if you wait too long to move him. Kessel is a downright awful player outside of his offense, but he's has so much offensive talent that he's one of the best wingers in hockey. Do you want to risk having a Kessel that has either taken a step back offensively or is a step slower? Because that is actually kissing contention goodbye, because you're not winning anything with a $7 million offense only player who has gotten worse offensively.
You don't trade Kessel to keep Bjugstad or anything like that, you trade Kessel for real value before you can't trade him at all. That's the justification to move him. You got 2 cups and 4+ good years out of him, when it only cost you (basically) a 1st and a prospect. You don't poke that bear of keeping him beyond his expiration date when you've already gotten so much out of him. You will definitely lose some of that good if you're stuck with a declining Kessel for multiple years.