He opted in most likely because no one would match that salary. Net negative to neutral value. Tradeable sure if the chips fall to need or for swapping salary inflation perhaps.
Needless to say but trades will work differently than FA. Kosta could have gotten this or a higher salary only from a team under the cap, but virtually any team can trade for him. He is the kind of player and contract that a lot of teams can find useful.
I had earlier posted a possible trade with Boston. Think they like Baynes, and he is a good fit in their system. However, I think KK is a clear upgrade over him. Haven't checked the numbers, but if they add someone like Semi Ojeleye, the numbers will probably work. Boston, if healthy next year, will look to seize the EC title from the Cavs (I don't think they beat them this year with their injuries), and if they can upgrade at center for the price of Semi, who has had an underwhelming rookie year, I think they might be interested. For us, Semi plays a position of need, and despite his rookie struggles, I think has the potential to be a good 3 and D player.
I'm not saying that we should trade Kosta, but that he is someone who has value, and can get us something. Depends on whom we draft, what we think is the potential of Giles, our long term plans for KK/Willie, our thoughts on JJ/Bruno/Nigel/pick to play the 3, etc. Overall, I'm happy that Kosta has opted in, because I think cap space has relatively lesser value for us. Kosta has been a thorough pro, who has played well in a backup role, and has maintained a positive attitude.