Thing is I dont see 2/3rds of the schools being relegated, if realignment continues the same way it has gone.
Right now Exactly 1/2 of the P4 teams are in the B1G and SEC.
The last round exactly 2 teams were essentially Relegated, Something like 3/100ths of the P5, but essentially kept their conference and their injury was at least muted.
Assuming the ACC implodes, We can also assume 4-8 of those schools go to the SEC and/or the B1G, at the same time another 4 go to the B12. Leaving 5-9 max to either rebuild the ACC or move down etc. Meaning about 1/10th or less of the original P5 total would be relegated at that point. Keep in mind how many have be elevated from G5 in recent years too.
If it continues that way, while I agree the B1G and SEC may pull further away, it would be hard for anyone in that new makeup of the B12 to claim serious injury, especially if the payouts continue to increase or at least remain the same. And I dont think a handful of schools would be enough to trigger something even at that point. Especially when at the same time a similar amount of G5 Schools were elevated in recent years. The only thing that would be an issue is if they make the playoff only available to the SEC and B1G but I am 95% sure they know that, so will avoid doing that for the foreseeable future.
Is it possible, sure. Is it likely, no.