As I said, this list is not just a collection of arms that flammed out as Cardinals' prospects or Major Leaguers, there are names on this list that ended up blossoming elsewhere. Sandy Alcantara is one of the most famous examples of that.
The number six overall prospect in the Cardinals system in 2017 and their third-best pitching prospect, Alcantara was a part of the package that St. Louis sent over to the Miami Marlins for slugger Marcell Ozuna, who had just finished 6th in MVP voting after slashing .312/.376/.548 with 37 HR and 124 RBI the prior season.
While you'll be hard-pressed to find many people who objected to giving up Alcantara for Ozuna at that moment, it clearly ended up being a major mistake for the Cardinal. In 2017, Alcantara got to pitch in 8 games out of the bullpen for St. Louis and wasn't anything special, but he never got the chance to prove what he could be as a starter.
The following year, in six starts for Miami, he posted a 3.44 ERA in 34 innings as a 22-year-old. He followed that up with 197.1 innings in 2019 with a 3.88 ERA. He progressively improved each year following until his Cy Young winning season in 2022 where he led all of baseball with 228.2 innings, posting a 2.28 ERA and spinnings 6 complete games. He's honestly the prototype pitcher the Cardinals would want in their rotation.
Alcantara has struggled most of this year, going 4-10 with a 4.28 ERA, but is still signed to one of the most team-friendly extensions in baseball and is someone I am sure the Marlins are betting on a bounce back from in the near future.
So why did the Cardinals give up on him? While we do not know how the Marlins felt about Luke Weaver, we do know the Cardinals had zero intention of trading Alex Reyes, and it's undeniable the potential that he had shown to that point. Jack Flaherty and Dakota Hudson were similarly rated prospects to Alcantara, so things could have been a lot different had the Cardinals been able to trade one of them instead.