You rarely trade away someone who is playing well for you unless there are contract issues or he requests a trade. Most trades involve prospects or usually productive players that are struggling in the hopes that they will regain their form with new scenery and teammates.
Almost all the biggest trades since the salary cap was instituted have involved a bunch of prospects for an established star, or a bunch of draft picks for an established star. Its rare for very good established players to move both ways.
Ericsson isn't good enough to bring in a bunch of prospects, so he's not really trade bait. If the Wings were going to make a deal for talent, it would likely mean moving someone like Sproul/Ouelette/Jurco/Tatar/Nyquist/Sheahan/Jarnkrok etc... Depending on who we moved I could live with that, but not for Sproul, I consider him untouchable.