I'd agree, except that he had pretty much maintained performance despite the loss in velocity until this year. And this year, he pitched through a back injury for several months--so I think there's at least some hope that he can rebound to 2011 form if healthy.
But even if he doesn't rebound, the Cubs haven't really lost much. He was still fine as a back end starter in 2012...and if he does that again, the Cubs can probably trade him at the deadline for as much as they'd give up for him now. I just think there's too much upside there to pass him up and sign someone like Liriano instead, who may or may not be any safer but definitely doesn't have the upside.