All the other P5s are going conference only, so all B12 10th games will be against G5 and FCS opponents if the B12 sticks to a 9 game conference schedule. Though it puts their conference SOS at a disadvantage to all the others, that's still the cleanest and easiest thing for them to do.
It's a pipe dream to think they'd scramble to add BYU. The ND insertion into the ACC is not comparable as ND was already a half member of the ACC, and ND already had half their games as ACC games.