This is probably a newsflash for you, but talk of getting out of the WAC and into the PAC 10 (and later the Big 12) started way back in the early 80's - probably before you were even born.
If BYU was already in big time football then BYU would've shunned the PAC 10, and put the WAC on its shoulders and carried it to the 6th spot in the power conferences.
So according to you, it should've been the Power 6, not the Power 5.