I mean to me it's really this simple. I'm down for trading shark because he's going to be what 29 next year? He has the tools to have a break out season similar to Scherzer. Hopefully that is from April through at least july next year and they get a kings ransom for him. The reason I think you trade him is that few pitchers are able to pitch into their 40's as effective as most pitchers pitch in their 32-36ish range. So, unless Shark turns into a player like randy johnson(talking longevity not his overall talent) and can have his peak at arguably 35-40 it just really doesn't jive with the cubs plans. On the other hand, if Shark is a more typical type pitcher you only have a couple more years at his peak. Maybe that's pushed back slightly by the wear on him being minimal. But you're probably looking at a 32+ year old Shark before the cubs are a perennial contender.
However, the return they get back really needs to be at least 2 quality players. If they do nothing else and just let him walk in 2 years they are going to get a 1st round pick out of the deal. A first round pick ought to be a top 100 player if the GM is worth anything. So, why trade him now for 1 top 100 guy and some throw in parts? If Shark gets off to a hot start like he did in 2013 who knows what he will be worth. For example, on june 29th last year he had a 5-7 record with a 3.34 ERA.
To me someone like KC or the pirates make the most sense. They are both in contention with a young core and can't afford to add top tier pitchers via FA. Both could be losing major starters(Burnett/Santana). Both have pitching prospects that would interest the cubs. The good young players they currently have are going to get more and more expensive so if they aren't going to go for it now when will they? Pittsburg in particular has Wandy, and Liriano coming off the books next year as well as Burnett at the moment. Shark would give them a nice 1-3 of Cole, Liriano and Shark assuming they intend to re-sign Liriano. KC could lose Shields next year but if they manage to re-sign him would have him and Shark at the top of their rotation and would still need some help.