My take is that Fitz does not have carte blanche job security. If we finish out 1-11, he is on the hot seat next year. Even 2-10 will probably yield that. If he has another double digit loss season next year, (assuming the two wins are interspersed and don't come toward the end of the season to give hope) one of two things will happen. Either a) there will be wholesale coaching changes b) Fitz will step down. (Far less likely) .
That said, 3,4, and 5 win seasons make things much murkier. Are we re-building? If we grab 3 conference wins, that will be the narrative. And enforcing change will be much harder.
One of the things that rarely gets mentioned in these conversations, is the fact that Fitz is a national media darling. No one has anything bad to say about him. He handles the media well., to his credit. The undercurrent is always, "Fitz had done a hell of job with what he has had to work." Few people, even football people, see things week to week to week, the way we do. They remember two valiant Big Ten title games, and now "this motivator of fine young men must be doing his darnedest to rebuild." In this kind of climate, Northwestern would get roasted for firing Fitzgerald without back to back to back ten loss seasons. This is not lost on the NU hierarchy.