As others have said, winners of BTL and ATL playing in a championship would be nuts. Our fans bitch about having Clempson as a permanent divisional opponent. Imagine the bitching if we were to get 2nd place in the ATL division and we get to see a Wake Forest play Clemson in the championship game....
My original response was more geared toward relegation as a whole and less to the specifics of an acc conference relegation, which is no where near as terrifying as relegation in P5/FBS as a whole with a top division of factories.
Our current system is not perfect, but it is getting better. In that same time span, there were no national championship games, the BCS natty game and the 4 team playoff, and soon to be a 12 team playoff. Expanding the playoff has been the right move to allow more teams a chance and for lightning-in-a-bottle programs to have a chance at a natty. The solution to the problem (good teams not getting a chance for the natty) is not to eliminate more chances for those teams in favor of consolation championships by using a relegation model. If you have a national championship caliber team (or even playoff caliber team) what good does a BTL/NIT championship give you if you can't play for the national championship? Especially if this one season was your chance, and you graduate all your star players.
you think it's been bad with teams like Cincinnati, ucf, Boise state are left out of a 2 or 4 team playoff? Wait until you have a 12 team playoff and an undefeated football power in a lower division passed over for a team in the upper division that is clearly worse.
the only situation where relegation might make sense is in and out of the Power 5 (60ish teams), but then you will still have the same problem like a ucf or Cincinnati not getting in.