Now, these contracts aren't terrible. In fact, the team and player probably broke even in the end. Neither side saw a significant advantage with the contract extension. Players like Carlos Martinez and Paul DeJong provided adequate value after inking their extensions, but they both fell off pretty quickly after getting a payday.
Paul DeJong received his arbitration extension after only one year in Major League Baseball. He placed second in Rookie of the Year voting, and John Mozeliak had seen enough to buy out the remaining arb years for the shortstop. His sophomore season didn't compare to his rookie year, but DeJong looked to put it all together in 2019; his K rate dropped while his walk rate rose, his slugging was at a career-high, and his defense was still premium. Altogether, DeJong tallied 3.7 fWAR that year.
The remainder of his contract is clogged up by high strikeout rates, an inability to get on base, and a dropoff in defensive prowess. DeJong was traded this past deadline to the Toronto Blue Jays for Minor League pitcher Matt Svanson. DeJong showed some promise early in his career, but he only had one good year during his six-year extension.
The other "bad" arbitration extension given out during Mozeliak's tenure would be the one given to Carlos Martinez in 2017. Martinez's extension was one in a long line of extensions between 2013 and 2019. He signed for five years and $51 million. The final three years of arbitration were bought out in addition to his first two years of free agency.
Martinez was fresh off back-to-back 3.0+-fWAR seasons with the Cardinals, and he would repeat that performance in the year immediately after signing his extension. The reason Carlos belongs in this category is the plummet he had after the 2018 season. Martinez averaged just over 1.1 fWAR during the length of his contract, and he even had a negative season in 2020. He was supposed to be the team's ace of the future, but after three strong seasons in 2015-2017, Martinez was never the same.