Look at the PAC 12 and how they have scheduled. With the exception of Colorado (for which I would love an explanation), since going independent BYU has scheduled games against every single member of the conference. The games against BYU have been among the best-attended of the entire season. It is a strong regional draw for a conference that, but for Utah and Colorado, would be separated from the rest of the P5 conferences by an entire time zone. They WANT to schedule games with BYU. This is especially true of the PAC 12 but it is at least marginally true for the other P5s. BYU adds more than it takes. If there is a large-scale, 65-team breakoff from the rest of the FBS or NCAA, they will want to include BYU.

Then there is the matter of anti-trust law. If there was such a large-scale breakoff, I'm not sure they could keep BYU out.