I'm not sure what it is that gives some players a pass for so long. David Montgomery has been consistently well below average, and while it's fair to blame a crappy o-line and poor play calling, I don't remember Jeremy Langford or Kadeem Carey getting cut such slack.
Montgomery spends way too much time stutter stepping when the Bears need a one cut back. Even a straight ahead runner might be better. Holes remain open for such a short period that by the time Montgomery accelerates, they are closed. This is another situation where I think it's time to give a different back some carries to pinpoint the problem. It's very possible Gale Sayers couldn't run behind this line, but it's hard to image Nall or Pierce could average any worse than 3.7 ypc and 2.1 ypc in the last 3 games.