In my opinion, a promotion/relegation system would be awesome and the best case scenario for lower tier teams. The average college football fan might be a little skeptical, but anyone who follows international soccer knows the benefits that a promotion/relegation system brings.
For one, hope springs eternal for the lower tier teams. Players and coaches know they can go play with the big boys on primetime television if they just prove how good they are on the field.That is a huge motivating factor. Fans and donors also feel like their contributions matter - like they are actually building towards something better.
Secondly, it makes every game matter more. How many fair weather fans have usually tuned out following a disappointing 3-7 start to the season? Probably most of them. More fans will care when they know the results of the last 2 games may just dictate whether their team is relegated down to the second tier or are allowed to stay up.
Unfortunately, promotion/relegation has about a 0.0000001% of happening. No way the tier one schools would ever allow it to happen. The only way we would ever get that is if we had a very powerful college football governing body that essentially forced it onto their members...and the NCAA definitely is not that.