How is a combined ACC/B12 really any different than separate? In that case you have a Super 2, a Power 1, and a G5.
If the SEC took FSU, Clemson, VA Tech, NC State and the B18 took Notre Dame, UVA, UNC, GA Tech, Miami, Stanford that would leave the ACC with BC, SU, Pitt, Louisville, Duke, Wake, SMU, Cal. Now your choices are to rebuild a new national ACC (can’t leave Cal alone) or half those teams can go to the better fit B12.
Would be ironic if the addition of Cal and SMU is what kills the chance of survival.
But we don’t know how this all plays out. I can totally see an Academic Colleges Conference made up of 18-24 Top 100 schools.