Why would a rebuilding team care about cap space? They can also retain on the contract as well to make things work in the right deal. They also let Datsyuk retire for nothing as well. Same with Lidstrom. I don't see why they would be unwilling to do so with Zetterberg. I also think you are over rating Galcheyuk. At this point his 30 goal season is looking more and more like a fluke. He doesn't compete hard enough away from the puck, and is trending towards a change of scenery type trade more than being the main piece in a trade for a star.
Kamiccolo may not be a red wings fan but he understands what is going on here:
Zetterberg is not just some old good player to us. He is not a "asset" that we want to trade for a 1st round pick.
Honestly I probably prefer we never trade him, and have him retire a life long red wing. Ask Ottawa how they feel about Alfredsson. thats how we feel about Zetterberg.
Look, we are likely to suck, we are likely to rebuild, and the main key in that rebuild is our own 1st round pick. Not other's draft picks. Also Mike Green is much better, younger, and a piece that if we traded, will leave our blue line as CLEARLY the worst in the NHL.
We are open to trading almost anyone. But Z is not a trade throw in. He is not a cap dump. If you said "i will take all Z's salary and you can have the 28th OA pick in the draft. I am inclined to say... no thanks. We like retiring our life long wings.
If Z asks out for a new team... different story entirely.