Well we know that actual stadium attendance has been declining for a couple of decades now. There are a lot of reasons for it, pricing and competition with other forms of entertainment at the top of the list. But the root of all of those problems, seems to be that college football just isn't engaging and retaining new fans at the same rate it's losing them. Younger people just aren't as interested in the sport, as we all were, when we became fanatics.
TV viersership is something else. There are more games than ever being broadcast, and although many cannibalize one another for individual ratings, the sum total of incremental viewership still seems to be increasing. There's even incremental viewership from the random Tuesday night MAACtion games that add to the total, compared to what was happening 20 years ago. That makes it hard to determine if fan interest is up, or if they're just broadcasting into a previously underserved market.