Mdot, no doubt that many of the first downs Michigan surrenders are to safeties or linebackers in man coverage but (1) that's not every attempt. On many passing downs, when the safeties shade properly and don't take false steps, those same players are in excellent position to contest the ball (or prevent the pass). Eyeballing it, I'd say that's half the time that they nail man coverage.
Also: (2) those passes are going for much more modest gains this year than last year. Every week, it seems like the longest pass allowed before the walk-one arrive is ~20 yards. And usually there are 0-2 of those.
I'm sure this is why no opponent is shaping their game plan to exclusively or primarily go after those guys. Because even if it's "the weakness" of the D, it's still filled with B or B+ players (Metellus, Kinney, and Gil/Ross) who either make plays or permit medium gains. It's very hard to consistently move the ball playing like that.