I think the best way to put it is this. Castro is one of the best players at SS in the league. However, the team that wins the world series rarely is the best collection of overall talent. It's about how all the pieces on that team fit together. Additionally, for me personally I'm never going to put an untouchable label on anyone who's not even the best player at his position. I love Rizzo and that arguably includes him.
I think the disconnect comes from people who want to trade him and those who don't in that those who don't seem to think those who do just want to sell him for whatever you can get. I mean let's be blunt here if the Dodgers called and offered Kershaw everyone here is taking that deal or you're a fool. It's entirely possible that the offers out there aren't worth Castro's value. If that's the case, you don't trade him. This was the same argument I made about Shark when people were talking about Aaron Sanchez and some 10+ org. prospect being the return. At that point I didn't see the value in trading him because even if Sanchez works out you're just replacing Shark and the chances of everything going right aren't high.
As for what's the appropriate value for Castro, I think you're talking a better pitcher than Shark. If Russell + stuff gets you Shark and Hammel then logistically Castro + stuff should get you a better return. If the Mets want Castro you likely start the conversation with Matt Harvey and then probably negotiate down to Zack Wheeler and Noah Syndergaard. Perhaps you can't get both but that's where my value on Castro would be. Wheeler's probably more of a #2 potential with Noah being obviously a #1. At that price, I'd consider trading Castro. However, I'm not trading him for Aaron Sanchez just as I wouldn't have traded Shark for him.