Maybe 30 years ago this was true, and still today regarding your MAC and CUSA schools, but I think it’s probably fair to say that Michigan, Ohio State, Notre Dame, and most of the schools who are driving the new era of college football have more fans supporting them with eyes and money who have no real purely institutional connection. Today, kids just grew up in Royal Oak, Upper Arlington, or with a grandpa who went to Notre Dame, and chose a team based on that and them being consistently competitive, even if they end up going to Grand Valley, Toledo, Trine, or nowhere at all.
I think the real barrier to the NFL ever actually taking over college football comes in infrastructure, politics, and purpose. The NFL is not going to want to bear the startup cost of a new league, be it stadiums, salaries, or just general upkeep. They’ve got a good thing going with the NCAA to NFL pipeline and I doubt they’re keen to rock the boat too much.
If the NFL did want to chip away at the college football foothold (and they might in the next 5-10 years), I could see them taking a more MLB-like approach to recruitment, expanding both the draft and roster size while reducing or removing the eligibility requirements, to accommodate NFL team that wants to select players right out of, or shortly out of, high school. And then those kids would have the option of either accepting something of a “futures” deal with an NFL team, or going to college. I can’t imagine the list would be long of kids who NFL teams are willing to take at 18/19 years old, but it would give them the option of stealing a Bryce Underwood or Archie Manning from the college game.
I just don’t think the NFL has any intention of reinventing the wheel, when they’ve already got the greatest money-making league in American history and no signs of slowing.