Let's get one thing out of the way first: the Cubs won the 2016 World Series. I bring that up for 2 reasons. First, that's awesome and I want to bring it up at every opportunity. The other reason is that it buys you a significant amount of runway to do whatever you want for the near future.
I'm not exactly sure why, but the Cubs are not good this year. At their current pace, they are an 83-win team, and they haven't given any indication that they are going to hit above their weight from July forward. There are some injuries, some demotions, and the return of the suck for the pitching staff. Compounding these woes, the farm team isn't what it used to be, and it's going to need to produce pitching at a pretty high clip going forward (or else the Cubs are going to have to pull some FA/trade magic). Next year, John Lackey and Jake Arrieta are both free agents; the former is cooked and the latter is one of the biggest FA question marks of the past decade.
Those aren't the only 2018 FAs, either. We'll lose our 4th OF (John Jay), our backup C/starting malcontent (M. Montero), and our best and 3rd best relievers (Davis and Uehara). All told, the Cubs are shedding $55 million in payroll next year, but go into next season with Kyle Hendricks as the ace, an aging/deteriorating Jon Lester as the #2, and Mike Montgomery as the #3. Anyone's guess after that.
I believe the Cubs should be selling off parts this year. The path to the NL Central is achievable, of course, and once you get there anything is possible. The problem there is that you'd probably have to BUY to get there, and that would be very irresponsible unless you're buying long-term assets at the deadline (and those are expensive). A team will pay something from Jake Arrieta, maybe even something good. A team will take on the rest of John Lackey's $16 MM payroll (prorated at the deadline, approx. $7 MM in cash savings). Don't laugh, it's true – someone would trade for John Lackey at the deadline. You'd get absolutely nothing but salary relief in return, but $7 million is not nothing when you need every dollar you can muster to sign Bryce Harper. Asshole antics aside, Miguel Montero could start at catcher for some team looking to improve at the deadline (probably an AL team that can stash him at DH a not-insignificant part of the time). Wade Davis would command a HUGE prospect haul – you could probably get more for him than you paid to acquire him. Koji Uehara would bring back something fairly interesting (a back of the top-10 in the org type). There's even a small (really small) chance that you could unload Zobrist, though that would be a salary dump in the same vein as Lackey.
If you could successfully unload all of these assets, you'd save $30 million dollars and you'd probably get a top-20 in the minors prospect and a pair of top-200 guys. That's 1/3 to 2/5 of an Alex Cobb contract, and perhaps a starting pitching prospect that you can slot into the back end of your rotation. When the price of that is the 5% or so of a WS you forgo by selling, it seems to be a no-brainer.
The counter-argument takes two forms. The first is that you might want to re-sign some of these players in free agency. If that's the case, go ahead and keep them (except Wade Davis – the Cubs should definitely sell Wade Davis is they sell anything at the deadline). The other argument is that the Cubs are a 13-3 run away from being 5 games up and looking like buyers at the deadline. My response to that is which 16-game stretch in the first 70 games of the season led you to believe that this team is at all likely to do that? The Cubs are a beast with 6 gaping bullet wounds (LF, RF, SP1, SP2, SS, RP). Why would sell a piece of 2018 or 2019 to buy a single band-aid?