The current format puts the BIG12 at a competitive disadvantage. It is being slowly starved of NFL-level talent as indicated by recent drafts. Other conferences can say to potential recruits "We make the CFP." So why would the B1G and the SEC want to do this? If (for example) the B1G has long term visions of adding some combination of Texas, Oklahoma, and Kansas (which I think it does) then the B1G would want to keep up the pressure. A continuing lack of success in the BIG12 encourages dissolution.
That said, and assuming that they do expand the CFP, then taking the top eight teams in the poll would essentially always assure that all P5 conferences were represented. The probability that at least one P5 champion would not be in the top eight is pretty small. That would solve the basic problem for the BIG12 and the PAC12, so there would be no need to have automatic qualifiers. A G5 representative that wasn't in the top 8 would certainly be seeded eighth and matched against the number one seed. That won't be a competitive game. I can see why the G5 would want it. I can't see why anyone else would want it.