Evaluators are often wrong but they're still more well-informed about prospects in the broadest sense of the term (as in all 30 teams) and they have no personal incentive to fudge the numbers one direction over the other (well, the good analysts, anyway).
As for Law, he's one of the better analysts out there. I'd put his track record against almost anyone. He's overly opinionated and a bit of a jerk at times but he's also good at his job. When he speaks, I tend to listen because there's useful information in there.
And anyway, I clearly said it was possible for the Brewers' rookies to all burst onto the scene but it's also unlikely and a pretty bad Plan A going into the season, in my opinion. That kind of decision is operating without a safety net.