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They stay in the ACC with their NC buddies.

I don't think being the top WCC program alone would be enough reason to stay.  I think you'd wind up seeing some sort of basketball merger of the WCC and Big West.  The only appeal to the WCC right now is the "better than they deserve" tv contract, solely due to the appeal of Gonzaga.  Without them, I doubt the WCC would get nearly the same type of tv deal next go round.
I also don't think staying put to be the top dog (as much sense as it makes) is how any school is thinking.  Every school when givne the option, is choosing to move up.  See Wichita to the AAC, Valpo to the MWC, Davidson to the A10, Hampton to the Big South, etc...  If they think overall it makes sense due to football, they'll do it, but I don't think staying would have anything to do with being the top dog in the WCC.

It's hard to say which side has more leverage as far as BYU and the MWC are concerned, even assuming Gonzaga does join, as it appears that they will.
The more I think about it, I think what we could end up seeing is a similar deal as Notre Dame with the ACC, where BYU guarantees 4-6 football games against the MWC in exchange for bowl access and joining for all other sports. The MWC is already at 12 for football with Hawaii as a football only school, and even if BYU were added for all sports, I don't really see the point of going to 14 in football with the best options being NMSU or one of the CUSA schools in Texas (UTEP, UTSA, Rice, North Texas). The MWC could still go to 14 for BB (w/o BYU FB) and other sports with another WCC school, Grand Canyon, or Denver (or just stay at 13, since they've been at 11 since the last realignment), which wouldn't cut into their football revenue. Of course, I wouldn't be shocked if San Jose State ever decided to give up on football and drop to the Big West.
That said, you're right that most schools want to be in the best league that they can be in. However, the WCC is comparable to the MWC in many sports aside from basketball (and they finished highest of all schools in either league in least year's NACDA standings: http://grfx.cstv.com/photos/schools/nacda/sports/directorscup/auto_pdf/2016-17/misc_non_event/D1final2017.pdf), and they're all religious institutions.
It will be interesting to see how it plays out.

Midlands Conference:

Iowa State
Colorado (from PAC)
Missouri (from SEC)
Arkansas (from SEC)
Texas
Oklahoma
Kansas
Kansas State
Texas Tech
TCU
Houston (from AAC)

Yes, Baylor is dropped from the P5 (obviously), with Houston getting the promotion.

PAC:

Also very clean, with only Colorado leaving for the midlands.

Washington
Wash State
Oregon
Oregon State
Stanford
California
Utah
Arizona
Arizona State
USC
UCLA

Man, I’d love that conference.

That's essentially JoePa's vision, with a few more additions to get to 11.

Man, I’d love that conference.

I've said it before, but the BigEast's vote on whether or not to take PSU was one of the most consequential things in the history of cfb.  

• Here is an article about the vote in which PSU's BigEast membership application was turned down by a single vote.  
• Here is an article from a PSU site that makes the case that losing out on BigEast membership was the best thing that ever happened to PSU.  

A BigEast with PSU starting in 1982 would have been a completely different thing than the "BigLeast" that we all made fun of back in the 90's.  I say the decision was one of the most consequential things in the history of cfb because it would have impacted the B1G (no PSU and likely no RU or UMD either), the ACC (who knows where FSU would have gone but if PSU was in the BE that would have made more sense), the B12 (no WVU) and who knows how many other long-term ripples.  

All of the original Big East teams were "saved" except Temple. 

In all these rounds of realignment, the only net change regarding P5/G5 status is that Temple was demoted, while Louisville, Utah and TCU were promoted.