When you get to the postseason, you can be forced to play a team like Baltimore, Indianapolis or San Francisco with excellent O-lines and well-coached, versatile running games. You need physical defensive tackles who can compete with elite offensive linemen, push people around and make plays. Beane is trying to win a world championship, not an award for most unused cap space. In a year they can get out of Star's deal and they'd instead pay Reader a similar percentage of cap space that they've already been paying Star, but for far superior play.
If they're going to keep Edmunds in the middle it might be a good idea to actually make 1-tech a position of strength rather than just passable.