Can I ask a semi-relevant question? Is Ian Desmond a tradeable player? Desmond hit .280/.331/.453 last year with 6.6%/22.1% bb/k rate a .341 wOBA and 20 homers 21 SBs and 4.4 UZR/150. Thus far that's been the peak of his career WAR wise and he is a career .269/.314/.431 hitter.
I'm guessing at least a few of you would say yes at the right price. The reason I ask is to remove the home-tinted glasses here. Castro's projected season this year is .288/.334/.470 with 5.7%/17.6% bb/k rate, a .350 wOBA and 21 homers, 2 SBs and a -9.4 UZR/150 and is a career .283/.323/.412 hitter. In any objective conversation the 2013 Ian Desmond is better than this current Castro because he has more speed, plays better defense and is as good of a hitter. The only thing Castro has going for him is he is 24 compared to Desmond who was 27. However, are those additional years really going to make Castro a better player and if so how?
Desmond was called up at 23 and had 1849 PAs before last year. Castro was called up at 20 and had 2617 PAs prior to this season. So, there's no more of the young kid jitters. By now they are worked out. Castro isn't going to start stealing more bases. In fact the opposite has happened. He's stopped. While that doesn't really matter I'm just saying he's not adding value there. Castro appears to have already added the power that often comes with age. So, I don't think that's another area he can improve. I mean he's not seriously going to hit more than 25+ HRs right? He really hasn't shown any measurable improvement defensively. His -9.4 UZR/150 is the worst of his career and specifically the range portion of it has decreased from a positive to a negative. That sort of goes in line with the steals aspect. So, tangibly the only other area that leaves is his average and walk rate. At .288 it seems really unlikely he'll measurably improve his average. I mean he might have a few .300 years down the line but we're not going to see a jump to .320+. And as far as his walk rate, it's identical to his rookie season.
In other words, this is what Castro is for better or worse. As for the question at hand about trading him, if you're going to entertain the idea of trading Shark why wouldn't you at least listen on Castro? Shark arguably does have room to grow and he's been as good if not better as a pitcher which is again, arguably harder to fill. And in the case of Castro, you potentially have a better hitting replacement near ready in Baez where as Shark you're hoping to replace him with the trade returns. This isn't to say you either A) have to trade him a la Shark situation where you're at a contract impasse or B) that you even should or C) even if you do trade him that it has to be today or even this off season. Maybe you are a better team with him. However, what if someone comes and offers you the Hershel Walker trade of baseball? You seriously going to turn down someone being franchise changing stupid for someone who's not even the best player at his position let alone the best player in the league?
Additionally, while Castro is clearly one of the better SS in the league that doesn't necessarily mean he's the best SS for the cubs. For example, some what ironically Theo when with the Red Sox traded Nomar. I don't think anyone can objectively say prior to the trade that Orlando Cabrera was a better SS than Nomar. Nomar was coming off of 7 seasons with the following WAR 6.4, 7.3, 6.3, 7.6, 0.3, 4.8, and 5.7. You're essentially talking MVP caliber player for the beginning of that. Cabrera had a 3.1 WAR season and a 4.4 season prior. However, by that point in his career Nomar was no longer a very good defensive player. Additionally, Cabrera gave the red sox a top of the order hitter with speed rather than a middle of the order hitter that Nomar had been. Remember that team already had Papi and Manny for the heart of the order. However, other than Damon they didn't have much if any speed. With Varitek and Millar they had more than enough power to offset the loss of losing Nomar. And maybe not entirely because of it but they did win the world series that year.
Now how does this pertain to the cubs? If you assume Baez and Bryant are the goods you're going to have 3 great heart of the line up hitters with Rizzo. Alcantara in theory provides you with 20 or so HRs. You also are talking about Schwarber as a possibility, Soler and Almora may have above average power as well. In a some what freaky coincidence what the cubs have almost none of is speed. Almoara isn't going to steal a ton of bases. Alcantara might. Baez can but he'll likely grow more power and lose that speed similar to the way Soriano and A-Rod did. So, I ask the question if that happens then do the cubs really need a 6, 7 or 8 hitter who hits 20 HRs and plays below average defense? Keep in mind that there are currently questions about Baez, Bryant and Alcantara's defensive positions and ultimately effectiveness. And outside of Almora there aren't a ton of guys in the system known for playing amazing defense. So again I ask a question. Would the cubs actually be better off with an Ozzie Guillen type(elite defense meh bat decent speed) who could be plugged into the #1 or #2 holes? An elite defender would mask some of the issues you may have in the infield with Baez/Bryant/Alcantara. It seems like they are going to have a crap ton of offense with or without Castro. Additionally, you presumably would get an elite talent back as part of the trade likely a pitcher.
Ultimately, it's about acquiring the best team not the best players. If that's with Castro, I'm fine with keeping him. However, I think the argument I just made illustrates at least one compelling argument where they are a better team despite getting rid of him.