Forget about the importance of the 5.75 million this year. What about in three years? Say some of the kids develop and we've suddenly become a contender with a bunch of rookie contracts (and Hertl, Meier, Cooch), and you need the money for a someone to put you over the top, would that money not be better used than on Karlsson who almost assuredly would be a liability by that point?
Also, for that matter, if his trading signals a tanking, would it not ease the pain a little if the team acknowledges it while playing more of the kids, which is what the plan probably is post trade deadline? I like the Sturms and the Lorentzes but they're not the future either.