It would be very rational for Idaho to rejoin the Big Sky. However, this is all about THE University of Idaho, the "flag ship" institution, the land grant school of the state. They don't want to be second fiddle to Boise State in anything including athletics. Many of us knew back in 1995 when UI and BSU left the Big Sky, that this was a bad move for Idaho. Moscow is a rural, isolated area (a beautiful area but isolated). They were in the Big West for a few years, then the Sun Belt, then the new WAC and now what? The UI needs to realize that athletics and academics are separate issues. In other word, athletically, they cannot compete with Boise State. Idaho would be smart to join the Big Sky. I agree with other posters that this won't happen however.
Here is part of their mentality.
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The belief that they get bigger game guarantees from being in FBS. Of course, ISU, a FCS school, has scheduled Oklahoma, Georgia, Nebraska (in the future I believe) and other big game guarantees and I am certain that ISU can bring a million dollars a year in when they want to be humiliated by a FBS school at $500,000 a shot. The UI also claims that they raise more scholarship money today (that might or might not have anything to do with being a FBS school). Certainly, the UI as a member of the WAC benefited when Boise State won the Fiesta Bowl, as I assume WAC schools shared in some of the money. They also would sell out the Kibbie Dome when they played Boise and they got a nice pay day (I assume) when they played BSU in Boise, but those days are gone.
Good luck to the University of Idaho.