Why watch college football? Because you're a fan of a team? I'm not sure about you, but most people I know who watch college football do it because they are a fan of a team, or it's their alma mater and they'll turn it on every once in a while, or their friends are fans and they'll join in to watch. For example, NDSU alums and fans watch FCS football but they don't tune in to FBS.
I know of very few people who are like "Oh yeah, this is the top matchup in college football so I guess I'll just watch it even though I'm not a fan of any team in the league." The people who do things like that are hardcore sports junkies.
It would be sort of like saying that people who are casual fans of the basketball would tune in to a top matchup in the G-League just for fun because it's the second best level of basketball. Not going to happen.
If a P2 league forms, people who have no association to the teams in it will not watch it. They'll watch their own team, and games from the league their team is in. If they watch any other football, they'll tune in to the NFL.