Auburn is proof enough of the volatility of CFB. A NC contending team one year, a poor team the next. Rinse, Repeat.
They are evidence enough of the fact that anything can happen, and teams should stay in their respective levels.
If Manchester United were to get relegated one season, that would seriously damage the franchise, and they might never recover. Relegation works in soccer because of the utter lack of parity. The elite teams dominate, and the rest struggle to stay afloat.
This would not work in CFB. Teams like WSU, OSU, UW, CAL, Colorado, would be decimated if they ever were relegated.