Bryce Harper
Fans know this situation all too well. Following the 2018 season, the Cardinals had the opportunity, like all of Major League Baseball, to add a Hall of Fame level bat to their lineup in the form of Bryce Harper, and decided not to.
With Marcell Ozuna, Harrison Bader, and Dexter Fowler already patrolling their outfield, the Cardinals decided they were better off turning their attention elsewhere. They also justified the lack of aggression with emerging prospects like Tyler O'Neill and Dylan Carlson, which also proved to be a fatal mistake.
Here's the problem with that line of thinking. Harper wasn't just a really good player who would be an upgrade for their outfield, he was a generational hitter in his mid-20s who the free agent market was not valued as he should have been. This was the perfect opportunity for the Cardinals to add a future Hall of Famer to their roster, and yet, they did not see the opportunity there.
Harper had a career .900 OPS from his age 19 to 25 seasons, making six All-Star teams and winning the MVP at just age 22. Typically when you hand out big-time free agent contracts, you're paying for past performance and hoping they can keep it going, but in the case of Harper, he was about to get even better.
Over the last five seasons with the Phillies, Harper has posted a .931 OPS and 149 OPS+, and maybe even more importantly, has been a historically great hitter in the postseason as well.
The craziest part to me, not just in reference to the Cardinals but also the rest of baseball, is that Harper was just 26 years old when that extension started, and the average annual salary on his deal is only $25 million. Even if you doubt he'll be worth that for the last two or three years of that deal, the first ten seasons are at a bargain rate. With the rate of inflation as well, that number will seem a lot lower come 2030.
As things currently stand, Harper is just the 22nd highest-paid player on an annual basis in baseball, and will only fall in those rankings as the years go on. Every team in baseball should have tried to sign Harper, but instead, he fell right into the Phillies' lap.