Checking what Renteria was doing at Castro's age, Renteria averaged a 1.66 WAR through his age 23 season and had already accumulated two sub-1 WAR seasons. It's definitely possible Castro doesn't take the next step in his game but he's still young enough that I don't think it's unreasonable to expect improvement. I'm not writing him off, but it is really disheartening to see almost zero improvement from him at the plate.