I have no real issue with trading guys at all. In fact I've always thought Baez would end up being the odd man out since he doesn't quite fit the the Theo/Jed mold. That being said I also think that the fact that so many in baseball think his ceiling is generational talent counts for something. I hope he's part of a deal that nets us a pitcher so we can stop talking about signing more 30 plus year olds for $30 million plus a year. You mention trades where trading prospects worked out but sometimes it doesn't. The Dodgers traded Pedro Martinez for Delino DeShields, Ryne Sandberg was traded for Ivan DeJesus and John Smoltz was traded for Doyle Alexander. Also your what if scenario for Brock for Broglio is kind of silly since that was a historically bad trade and the scout that had identified Brock quit the same day. The trick is knowing the guys to trade. I don't think the the Cubs know enough about any of their guys right this minute to make those decisions. Hopefully they'll figure a ton out this year and we won't even have to talk about this any more.
For every Pedro/Sandberg/Smoltz I'm willing to bet I can come up with 4 trades that didn't work out well on the prospect side. I also think it's a tad disingenuous comparing Baez to those players because Sandberg and Smoltz had no real playing time in the majors and Pedro had 100 or so innings of great pitching. Baez on the other hand has the highest k rate of any player ever with 200 PAs. If people wish to believe in his potential that's fine but you can't just ignore that as a astronomic concern. It could be growing pains or it could be real but that's sort of my point. Why gamble? You're first gambling that he improves. You're then gambling he improves enough to be worth it. I'll take a guy like Denard Span has been the past 4-5 years over that potential any day. That's not to say I'd trade Baez for Span now given Span is 30 but rather a young player that's consistently good but not necessarily a star.
As for the Brock thing, the point was it's only a bad trade because they got nothing out of the other side of the trade. It's not a bad trade because Brock went on to be a HoF player. In fact, I'd argue he'd never have been the same player with the cubs because both his steals and his OBP improved immediately after being traded which should tell you the Cards saw something the Cubs didn't.
People get greedy about having all the star players. I've said this before but winning championships isn't about having the best players it's about having the best team. The giants last season had 5 players over 2.5 fWAR with those being Posey(5.7), Pence(4.7), Sandoval(3.0), Crawford(2.7), and Bumgarner(3.6). The cubs presumably already have 2 for the foreseeable future with Rizzo(5.6) and Castro(2.9) and may have more with Arrieta(4.9) and that's without even mentioning the prospect army. What kills teams is when they get nothing out of a position which is why I believe this for the lack of a better term "all or nothing" approach with Baez is the wrong one. Give me the sure thing as a 3-4 win player any day over the hope someone is a 4+ WAR player. This is especially true when you're already talking about dealing from a position you're strong at.
