This is a pretty simplistic way to look at things and is extremely short sighted. To throw up a pretty simplistic example of this, let's say the cubs traded Kris Bryant for David Ortiz. Ortiz is clearly the best hitter in the majors this year with his 180 wRC+. So, by your definition that would make the cubs 1% better. But the obvious flaw here is he's retiring after this year. Now admittedly that is a stacked example to prove the point I'm making but the same logic applies. Trading young players for in their prime players is always a trade off for good over a long period vs good now. And if you do that enough then you have no one who's good next year.
The other problem with trading Schwarber that I haven't seen people mention is the utter lack of left handed power in baseball right now. As of today here's the guys with more than 15 HRs; David Ortiz, Anthony Rizzo, Robinson Cano, Chris Davis, Jake Lamb, Carlos Gonzalez, Bryce Harper, Jay Bruce, Daniel Murphy, Kyle Seager, Brandon Moss, Freddie Freeman, Rougned Odor, Corey Seager, Curtis Granderson, Joey Votto, Michael Saunders, and Jason Kipnis. It's not a lengthy list and a lot of the guys are either in their prime or on the down slope.
If you're going to trade Schwarber, you better get at least equal value back and every baseball person out there has said that from a baseball standpoint you would never trade Schwarber for a reliever. If you or the cubs still believe their bullpen is weak that doesn't mean you shut down trade discussions for other relievers. You can easily target other relievers. But just giving up one of the best young talents in baseball because another team draws a line in the stand is horrible negotiation.