Nothing, really. It's a fair contract for a free agent with his skillset, and he's going to be playing in a home ballpark that will play well to his skills.
Castellanos is a good hitter, but not a great one, and his only distinctive skill is power. But the Cincinnati ballpark favors a power hitter, and Castellanos is likely to increase his home run output playing there. The Reds realized this, and since they could sign him without upsetting their salary structure, they did so. They're trying to contend now and he should help.
The only one that I think it’s not realistic for someone to break is the 511 career pitching wins amassed by Cy Young. No one else, even in Young’s own time, ever had more than 417 (Walter Johnson), and the most by a modern-era pitcher is the 355 of Greg Maddux.
Young pitched in a very different era, where pitchers didn’t go all-out on every pitch, and by all accounts he was a freak of nature anyway—a guy who was able to pitch every other day if necessary, and who threw harder than nearly all of his contemporaries. His win total is sometimes derided because he moved to the new American League in mid-career, and the AL wasn’t quite as strong as the NL. However, Young was one of the best pitchers in the NL already, and as far as the AL being weaker, Young pitched his Boston Americans (known as the Red Sox today) to the first World Series title in 1903. So if they were weaker, Denton True Young apparently was good enough to bridge the gap.
Even if Cy Young hadn’t switched leagues, he’d still be the all-time wins leader and probably have something like 480 career victories. He pitched until he was 44, and even at the time of his retirement had only slipped to more or less league average performance. Basically, he was the Nolan Ryan of his generation, only he was a better pitcher than Ryan was.
Bill James once wrote that a record is safe at any given time if it consists of 23 or more seasons of contemporary league-leading performance. The most wins in a single season by an MLB pitcher in the last five seasons is 22, by Jake Arrieta in 2015 and Rick Porcello in 2016. A pitcher who won 22 games a season for 23 years would have 506 career wins.
Even Pete Rose’s 4,256 career hits is more attainable than this. In fact, there’s a pretty strong argument that Ichiro Suzuki could have broken it if he’d played his entire career in MLB. And presently, Mookie Betts is averaging 196 hits per full season. If he could do that into his 40’s, he could easily challenge Rose (196 hits for 23 seasons is 4,508). It takes a great player who collects a lot of hits (like Betts) who starts his career at a very young age and who then ages exceptionally well, with the desire to play a long time, but it is possible. Derek Jeter had 216 hits at the age of 38 and was at 3,304 for his career, but when he lost basically his entire age 39 season to injury and came back as a much diminished player, his chance was gone.
I could go into detail about how many of the old records are untouchable, and just echo the dozens of answers already here, but I won’t.
I’m going to talk about a recent one that is unbreakable for its own reasons.
Fernando Tatis smacked two grand slams in one inning off Chan Ho Park back on April 23, 1999 when the Cardinals squared off against the Dodgers