The NBC dollars are less than schools like Purdue and Illinois make from just being part of the collective. ND is not independent because of NBC money being so much, rather, NBC is the means/vehicle to the ultimate end: Independence, by and for itself.
Independence for ND allows for them to play seven games not dictated by a conference, including games in California and series with Texas, Georgia, Ohio State, etc..
But more importantly, it is part of ND's identity. It sees itself as a national school, that plays and recruits nationally.
Conferences are regional by nature and limit that.
ND has never been in a football conference in the 127 years it has played football. Why give up that status?
ND has only had less than two losses once (2012) since 1993. Is the de facto "threat" of being "left out" of the playoffs really that important to jettison ND' s identity and unique status?
The playoffs are a very rare occurrence. Independence is all of the time.