Starting pitching is so talent expensive via trade that it just makes more sense to buy through free agency.
That's more of an interesting debate I think because I'm not entirely sure I agree. I'd argue the problem with starting pitching trades is the quality of them often never materializes. For example, how long have the A's been "trading" gray? But if we're talking purely value I'd argue the Quintana trade was reasonably priced. Sale was a big price to pay but he was also probably worth it. Where I bristle a bit when talking about relievers is you really don't know year to year who's going to be good. For example, Rondon was one of the best relievers in baseball for much of last year and then just fell off hard after an injury. Grimm 3 years ago was dominant and looked like a big piece but hasn't had it since. Chapman hasn't been the same this year either.
I think that's what largely makes this discussion interesting. Starters are usually fairly safe bets once they are in the majors. You pretty much know what they are usually outside of injury concerns. And pitching in general is so hard to develop that while paying starting pitching prices hurts, overall I'd argue many starter trades end up being plus value for the teams buying MLB pitchers. In terms of buying via FA, I think that's a bit more hit/miss. For example, do you want David Price on his deal right now? He's been ok but no where near the cy young money they gave him and presumably at age 31 it's only down hill from there.
In terms of how you acquire bullpen help, that's a tough idea in my eyes. Obviously I'm not a fan of paying for it in trade. I'm also not really a fan of paying FA relievers. And drafting and developing pitching in general is just hard for so many reasons. So, I'm not 100% sure there is a reliable way to develop relievers. What I would like to see the cubs do is more focus on guys with big arms who could eventually become something useful. For example, I really liked them picking up Jack Leathersich. He's not quite worked out like you would hope but if he gets some things figured out he can be a really impactful piece. Unfortunately, I don't feel like the cubs have done enough moves like getting him. And obviously there's rule 5 type arms you can try to develop.
The next two years should be interesting. I believe both Rondon and Strop are FA's after this year. Davis definitely is. Think Koji has a 2 year deal. Duensing is on a one year deal. So, the cubs are going to need a lot of help there. Maples is probably good enough that he'll be useful there. He could be one of the better relievers in baseball if he can command his pitches better. His stuff is excellent. And obviously you'll have Edwards and now Wilson plus likely one or two vets. But they are still going to need some arms. I hope they at least try to develop some of their own as say the long man type guy rather than going with more of the Cahill/Duensing type approach.
