Agreed. Signing Castellanos makes no sense. I know its been beaten to death by the Chicago media, but Schwarber was BETTER than NC from the day he was acquired through the rest of the season. Is it sustainable for Kyle? I don't know. That same question can be asked of NC. The two players are very similar (both bad in the OF), and this lineup is already heavy from the right side. How many lefties hit 38 homers last year? Kyle is also younger and half the cost.
The cubs doing a half-ass rebuild won't work like you said. There's no scenario where the cubs trade KB for prospects and compete in 2020. You're wasting other guys primes. By the time these prospects we get in return for KB are ready, you're going to have less productive Rizzo, Javy, Darvish, Hendricks, etc. I'm not saying the cubs should or shouldn't move KB. But if you are going to move on from him, you might as well go full-reload and move Willson and Quintana too. If Chatwood or Kimbrel regain some value by the deadline, they should be dealt. Lester comes off the books after 2020, and you should have a very large bankroll to play with and potentially a new wave of prospects. Cubs have 4 of 5 legit prospects (most are years away) and not much after that. KB/Willson/Q would bring back a massive haul. Its always a crapshoot but at least they can sell the fanbase on this vision. Right now they feel stuck and I think we're all ready for a new chapter.
Off topic but I think Ian Happ being on the roster every day for a full season will be a significant improvement to the position player group. If you look at the # of AB's we gave Almora, Descalso, and co it will make you vomit. He needed to work on his swing in Des Moines but he looked much better after his call up. Happ and Schwarber definitely get more hate than they deserve, the expectations were just set too high.