CU isn't any more a rival than Oregon State is.   You are correct on this.  The games that get the most discussion are USC, ASU, Washington.  

Nearly every Utah fan I know still considers BYU the rival.  The only difference is that now once the game is over, it is over.  The focus is shifted immediately to the PAC-12 South and the PAC-12 Conference.  This lessen's the intensity of the rivalry game.  But how could it not? It doesn't have any implications for the conference, whereas a loss to USC, like this year, is like losing two games because of the negative impact on conference standings.

If BYU were in the BIG 12 it would be exactly the same.  It is a really fun game for that week.  One of the best games all year, but it only lasts one week.