I think millennials are the least likely to pick up the slack, to be honest. They know they have plenty of other entertainment options, and they have far less history with the program than the folks you describe. They had a glimpse at greatness in 2012, only to see it all come crashing down--first on January 7, then in 2016. But they haven't seen national championships and historic games every year and a program that strives, above all else, for excellence. Why would they choose to pay thousands of dollars to uninteresting games when they can easily get tickets to the one or two games of interest for far less?
I haven't been to a game on campus since I graduated from law school (five seasons ago). And I've only been to three games total in those five seasons (two in 2012 and one in 2014). I can't imagine buying season tickets, even if I lived much closer and could reasonably expect to travel to ND for six games each year.