Only way that a relegation system could work is if you leave the P5 conferences alone, and create a best of the best 6th conference where those 6 conferences play top level. Still no reason why the P5 would allow it since it gives another conference a seat at the table, but at least they have to perform well. Also would pretty much have to get rid of all the G5 conferences for football and have the teams left behind play regionally in 4 divisions, maybe 2 conferences of west and east. That way if you want to promote two teams, the best in the west and east move up presumably to take the spot of the worst teams in the west and east of the 6th conference. The G5 conferences would have to stay the same for all the other sports and tv contracts would be a mess.
West: Boise St, SDSU, Colorado St, UH, SMU, Tulsa, Memphis, UTSA/LA Tech
East: USF, UCF, Navy, Cincinnati, Northern Illinois, Temple, Western Michigan, Toledo/Marshall/Ohio
Never going to happen, but interesting to think about.