The unfortunate truth is that there will never be this type of list again without mentioning Sandy Alcantara.
Sure, he had only shown brief glimpses of promise during his tenure on the Cardinals, but it's hard not to feel like the Cardinals just cut bait way too early and are now forced to pay the price.
Alcantara had joined the organization as an 18-year-old all the way back in 2014. He quickly established himself as a major strike thrower who had some minor control issues, but did a great job of limiting home runs. Through and through, he was a fast riser through the system.
Just a few years after his professional debut, he was on a big league mound as a 21-year-old. The only caveat here is that the Cardinals only used him in a relief role in 2017. He performed fairly well, posting a 4.32 ERA and 102 ERA+ in eight games and 8.1 innings of work, but his 6.04 FIP suggested that some of this success was based on luck.
As we all know, the righty was flipped to the Marlins in the 2017-2018 offseason. His rise to stardom didn't take long for his new club, and he has since made a pair of All-Star Games and is the defending NL Cy Young Award winner. This one will forever hurt, because the second he was put into a consistent role as a starting pitcher, he blossomed. He has improved more and more in virtually every full season he's had in the big leagues.
When all is said and done, Alcantara will likely be one of the biggest "what ifs" in Cardinals franchise history.