Was trading Torres for Chapman short sighted? Obviously you haven't read my thoughts but I said it was at the time and I'd argue it today. Think you can make a strong case that the 3 blown saves Chapman had in the playoffs were as negative to the team as that one strong game 5 run he's largely remembered for. Not to mention that's always going to be a bit of a disingenuous way to have the discussion because it's assuming there weren't other options out there. For example, Mark Melancon isn't Chapman. No one would seriously suggest he is. But he cost Washington Taylor Hearn and Felipe Rivero. Hearn's a 45 grade prospect in A ball. Rivero is an interesting reliever but neither of them is Torres. There was an article that came out talking about the trade after it happened that suggested the cubs own front office thought the Chapman trade took them from a 20% chance of winning it all to 25%. Obviously there's debate to be had on whether or not a 5% chance is worth it. Personally I say no.
Ultimately, my view is that if they can't re-sign him you deal him because in this particular trade market you might get far more value than you other wise would. Humor me for a moment on these prices as we don't really know what the will end up being but for example let's say you can trade for Archer with Soler, Happ, Montero(+offset money), Candelario(or equivalent prospect) and 2 or so 45 grade prospects. And let's then say you can get Francise Martes and David Paulino plus some lower level prospects from Houston or equivalent high minors pitching from someone else for Arrieta. You then line up Lester, Archer, Hendricks Lackey and Montgomery as your starting 5 with Martes and Paulino in AAA in case of injury both of whom could potentially be frontline starters. You've effectively turned one of your weaker areas(young pitching) into one of the stronger elements of the team having 4 guys you control for the next 5 or so years near the majors. The upshot of that is not only that you get younger but it also covers the loss of Arrieta after 2017 and the loss of Lackey without costing you much of the overall value in your system.
As I said it comes down to betting on 1 year vs betting on a larger window and I'm always going to take that larger window. Even if the cubs win it in 2017, you're going to then be in a situation where you're having to replace lackey and if we assume they make that same Archer trade they then have very very little to move and the FA market is very thin. Additionally, you're then hoping Underwood finally breaks out or else you don't really have anything in the high minors that is MLB ready.