Not winning a National Championship does not make a season a failure. The fact that there isn't a National Championship at the end of every season doesn't make a season a failure, and certainly doesn't mean ND shouldn't set its goal every season towards winning one.
Being happy with three to four losses every year is the path to true irrelevance. Notre Dame has the resources (financial, logistical, and otherwise) to win National Championships in football. It's equally important to have the will, and the quickest way to lose that will is for the fans to shrug their shoulders and decide, "well, it just can't be done".
Most seasons fall short of goals. If your goal is three losses, you're in trouble.