then one of two things happens

          Either the PAC12 backfills from the Big 12 OR
          The Big 12 absorbs several of the PAC12 teams (probably Utah, Colorado, Arizona, Arizona St)

But the bigger issue is what happens back east as the B1G and SEC strive for supremacy.  Probably means the SEC poaches Clemson, Florida St, maybe Miami and one other bringing them to 20.

And the Big 10 responds by adding UNC and Notre Dame (who they can get now because of what comes next) and goes to 20.

And now there's only 2 conferences and they effectively become the AFC and the NFC, separate and distinct from the rest of college football, playing by their own rules and only counting games between one another.  Could even have a playoff within each conference and then a final championship between those two champions - a super Bowl game if you will.  And so we end up with a formalized split

SEC + Big 10
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Remnants of the former P5 (where the most interesting team left is Oklahoma St or Virginia or Utah) filling the role that the G5 used to have or merging down to effectively be the new G8 (or 7 depending on consolidation)

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FCS schools