The equation for Castro and position movement doesn't hinge on all three of Baez, Russell and Bryant being better infield defenders. It hinges on Bryant' ability to play third. It's pretty much a given that Russell and Baez are better defenders, long-term. If Bryant can handle third and you're moving him to LF to accommodate Castro, you're diluting some of Bryant's value (and Castro's too, by moving him off SS). Alcantara is a wild-card - he offers insurance in case Baez or Russell flame out or Bryant can't play 3B, which makes the risk in trading Castro more acceptable. And I certainly don't think it's a lock he won't be a better offensive player, long-term. None of this means you must trade Castro. But it does mean you can, with a reasonably low amount of risk. It's a matter of whether his value as a shortstop is enough to fetch the right pieces in a trade. If you can get the maximum value out of everyone playing their maximum value defensive position, that's ideal. But if not, you can move guys around when the right deal isn't there.