I think it may be a little soon to decide that Castro will be better than Sandberg was.
At the moment it is still premature to say Castro will be better than even Dunston was, or Kessinger or Beckert or Manny Trillo, but, just to try to put things in perspective...
Castro turned 21 last month and now has 224 career total bases. That is two years younger than Sandberg reached that number.
Castro had an OPS+ of 97 last year. Sandberg was five years older before he did that well.
Certainly Castro could tear a ligament, develop eye problems, forget how to recognize a curve ball, decide he should swing for the fence on every pitch, start smoking crack, gain 45 pounds, or set up permanent residence on Rush Street. Any of that is possible.
But we need to appreciate just how truly special a talent this guy is.