I think fans on this site, not unexpectedly, value Brewer prospects than most other sources do.
Maybe outside Chourio, most of the players you cite have legitimate concerns about their game: Frelick-lack of major league power, Misiorowski-lack of major league command, Black- fringy major league arm, lack of MLB caliber power. Face it, none of these guys are blue chippers in the vein of players like De La Cruz , Kris Bryant, Carroll, Bryce Harper where the only question is how big a star they would be in the majors.
This is why most of the pundits rank their system in the middle of the pack: lack of impact talent close to the majors and lack of overall depth. Lets not forget, less than a year ago Brice Turang was one of their top prospects (a player so far looks like a slick fielding no hit infielder),
Point is: even if Rick Hahn inexplicably decided to trade Robert, he would have much better packages of players to choose from than anything the Brewers could muster.