The chance that ND would have an opening in its schedule due to a late cancellation would be extremely slim, so why bring up the possibility?
During most years, our schedule is pretty much set in stone: 5 ACC teams, USC, Navy, Stanford, MSU, and Purdue. That's ten out of twelve games.
If one of those teams wanted to do a two year hiatus (like MSU in 2014-2015), ND would be aware of it several years in advance, and would have time to schedule a decent opponent.
If an ACC team that was scheduled to play ND decided to move to another conference, ND and the ACC would have at least a few years to fill in that spot with another ACC team.
Unless Miami were to get the "death penalty" from the NCAA, there's no reason why any of those teams would do a last second cancellation.
That leaves only two games on the schedule. I don't see the Texas games being cancelled, and it's unlikely that we will cancel the BYU games either.
Even if a situation arose in which another team bailed on us on short notice, there's no shortage of Big East, Mountain West, and Conference USA teams that would gladly play us.
Ironically, it will be ND that may have to do some last minute cancelling in the next few years. Assuming the 5-game ACC arrangement starts in 2014, ND will have to cancel either the Temple or Northwestern game.