It was popular before this season to suggest the Cardinals lost the Jason Heyward trade. The conventional wisdom went something like this:
Heyward was a one year rental for which we traded long-term value in Shelby Miller. Yes, Heyward provided more value in his 2015 season than Shelby, but Miller's value increased, while Heyward's was consumed. After the season, the residual value of the trade was a draft pick (which yielded 17 year old Dylan Carlson) and a broken reliever who might never see an MLB mound again. The Braves, on the other hand, had a long-term asset that was converted into Ender Inciarte, Dansby Swanson, and Aaron Blair. So, by the transitive property of player swaps, the Cardinals got totally screwed.
But not really. First off, let's look at the counter-factual reality in which the Cardinals hold onto Shelby Miller. Was that alternate-universe Cardinals team appreciably better than our Cardinals in 2015? No, clearly not.
Shelby Miller was quite good in 2015, but Jason Heyward was far better. Heyward's 6.0 fWAR were almost double Miller's 3.4 fWAR. The starts Miller would've replaced on the 2015 Cardinals staff belonged to some combination of Carlos Martinez, Jaime Garcia, Lance Lynn, Tim Cooney, Tyler Lyons, and Adam Wainwright, all of whom were good to great. Those 6 pitchers combined for ~13.2 fWAR in 128 starts, as compared to Shelby's 3.4 fWAR in 33 starts. That's almost exactly the same on a pro-rata basis. And while Grichuk/Piscotty/Pham would've also provided high-value, low-dropoff offensive replacements for Heyward, it's hard to argue they would've provided his defense in RF. The Cardinals were almost certainly a better team in 2015 thanks to the Heyward deal.
So what, you might be thinking? The alternate universe Cardinals would've still been good enough to make the playoffs, and then they could've traded Shelby Miller to the Diamondbacks for all those sweet, sweet prospects! But would they have? Imagine yourself as alternate universe Moz. Entering 2016, you have a deeper pitching staff, it's true. But is Miller on the trading block? Probably not. Alt-universe Lance Lynn still needs Tommy John surgery. You no longer need to sign Mike Leake (or make an offer on David Price), but that doesn't mean you're a seller of pitching. Most likely, you thank your lucky stars that you still have Shelby Miller, and you stand pat in the 2016 offseason. Which means, you get...the 2016 SHELBY MILLER IMPLOSION EXPERIENCE!
Shelby Miller has been astonishingly bad this season. He's had just 3 starts with Game Scores over 50 (average), and just *one* Game Score over 60. To use a different metric, he has 3 Quality Starts, out of his 18 major league appearances. He threw 90 innings with a 6.90 ERA, and he's allowed his worst wOBA of the season in September. This 2016 Cardinals team has *barely* been good enough be in playoff contention in 2016. If we had 90 innings of Shelby Miller, we might be below .500. And we'd be hoping and praying for a Shelby Miller bounce back season in 2017 (or looking for Moz to sell lower-than-low on a pitcher we all grew to hate).
Trading players is a bit like trading stocks. And Shelby Miller is like Yingli Green Energy - a former high-flyer that crashed, recovered, then cratered. We sold during a moderate lull in his value. Then his market recovered, and we felt dumb, as the shares we cashed in tripled in value. But now, Shelby is plumbing new lows, and we should be grateful that we got out while we could still sell and weren't left holding the bag like the Diamondbacks.