The Big East was relatively new to college football. One original member of the Big East, Temple, didn't make it to the P5. Three teams that joined in 1991 made it to the P% : VT, West Virginia and Rutgers. One team that joined in 2005 made it to the P5, Louisville.
So even ignoring the issue of conferences disappearing, the question becomes could there ever be another Big East that effectively becomes P5 or feeds most of its teams to the P5.
I don't think it is a "fact" that it could never happen again. Given that there are teams like UCF, Boise State, BYU, Houston, Navy, Memphis, Cincinnati, not in the P5, the possibility is there.
There is a big difference between saying unlikely in the next 25 years and a "fact it will continue indefinitely". Times change. Just ask the Ivy League.