This is a terrible idea every offseason when it is brought up, and it is still a terrible idea. Some things to consider:
-Even greater disparity. Since the EPL was formed in 1992, exactly 4 clubs have won the title. The top four clubs have held the top 4 spots in the league 75% of the time (58 out of 76). In that time, more than a dozen schools have won AP national championships in football. In less time than that, Boise St. has gone from I-AA to a top 5 ranking.
-Relegation is a negative feedback loop. The worst teams in the league are is essence punished by reducing their revenue and access to TV. This makes improvement difficult, and causes the gaps between the "haves" and "have-nots" to widen. Often, the drop causes the team to suffer even further due to loss of players (compare to decommits in CFB - think of a player who commits to SMU but changes his mind after they are relegated). Examples:
http://www.thisislondon.co.uk/standard-sport/article-23664566-premier-league-casualties---clubs-that-have-struggled-since-relegation.do-It can destroy rivalries. If UT and A&M don't want to play each other anymore because their panties are in a wad, that's fine. But I want to see SMU continue to play TCU, Houston, Baylor, A&M, etc. Those are the teams that I get excited about. I don't want to be stuck playing UNT, Louisiana-Monroe and UTSA because we got relegated. An example is West Ham, which has a big rivalry with neighbor Millwall going back more than 100 years. However, they have hardly played in recent years because they haven't been in the same division (until this year).
-The financial model doesn't work. The only way for lower division teams to improve themselves is to get a better caliber of player. That primarily has to happen by offering those better players money to play for you. The NCAA frowns upon that sort of thing.
And in the interest of full disclosure, I am an EPL fan. It just isn't the right system for college football.