The issue is if the Alliance does 8+1+1 (which is far from certain) the Big 12 is going to have more trouble playing good teams in the non-con. I think in that case, playing 9 conference games and trying to play an SEC team just about every year is going to be the move.
Several (4?) ACC teams have rivalry games with SEC schools, plus there's Cyhawk. Also the PAC has two less schools than the ACC and B1G. Plus I wonder how Notre Dame fits (don't they have an ongoing rivalry game with USC?).
tl;dr: it will still be possible to play games against Alliance schools, but probably less opportunities going forward.