NFL losing viewers has nothing to do with CFB gaining viewers. They aren't even competing with each other.

NFL is losing viewers because of faults they have and stupid things they are doing, like the sterilization of the game and no fun league. Though I suspect most of it comes from cord cutters and illegal streaming, but those numbers are hard to measure.

In addition you things like the debates overlapping and the fact that most primetime games have been utter shit and containing bad teams this year, prime time is really hard to predict before a season start if multiple good teams shit the bed and stop playing good football.

CFB on the other hand are doing a lot of things these days, though they are worse of in regards to the amount of commercials. Hello 5 hour broadcasts on CBS. At least the NFL is a little consistent i broadcast time.

In short, while it's good for CFB that viewership is increasing and bad for NFL that their viewership is down. The two are utterly unrelated from each other.