The team can only give themselves a shot at the playoff if they win out. You are kinda assuming that since Miami and A&M beat ND that they are somehow going to continue winning and just be locks for the playoff.
Conference play is starting up for real now, Miami could easily lose to FSU (Cristobal hasn't Cristobal'd a game yet, even though he tried with ND game 1), A&M is always good at losing games later in the year. The SEC can certainly have upsets out of nowhere. ND can lose to USC based on what we've seen the first few weeks on defense, we have to see if the defense continues to improve prior to that game.
This year of college football has already been a lot of fun. ND needs Miami and A&M to keep winning, and ND needs to keep blowing everyone out. The committee will need to be able to look at what the team is doing, how much it has improved, in order to determine if they think ND will actually have a shot in the playoff. If A&M manages to be Texas 8&4 again this season, and Miami fails to win the ACC again this season, those losses, while close, won't be "good" losses anymore, and ND will be very lucky if they are able to sneak into the playoff were that to happen.