This is a bad take. 1st of all you aren't qualified to say if they are, or are not, ready for the NFL. 2nd it's hard to imagine Prater or Sash improving thier stock or game. Prater is 1st team all B10 this year which is quite an honor. Sash is a 3 year starter on D who has peaked physically and excels at the mental aspect of the game. If the scouts tell them they are sure fire draft picks and they have little to improve on then it's hard to come up with reason why they should leave now, especially given the potential labor unrest in the league.
McNutt who's only played the position a few years is really the only one with anything to gain.
Lastly the "they should stay for their education" argument is pretty stupid as well. If they get drafted they will make more than enough money to go back and finish school if that is a priority for them, and if it's not chances are they aren't putting enough effort toward thier school work to earn a meaningfull degree anyway.