If Notre Dame had a plus-one of OK/OKSt/Michigan/Ohio St., I think they get in with only a few points loss to #1 Clemson this year. The other team would be motivated to play to get into the best New Year's day game with a win. With a ND loss, then the non-selection for playoffs is a no-brainer.
But on years with a one-loss champion from a P5 conference, this could be a critical game for that team -- not necessarily Notre Dame or BYU. Just one more quality win is all that it would take for that team -- assuming best of ND and/or BYU are top 25.