They really haven't had a span of down-years to bring them down. As a Utah fan who grew up in the 80s (And regularly attended games during that span), I had to mellow out in my fandom for survival, or my BYU fan friends would have torn me up.

Most of my BYU fan friends growing up had also never experienced a "down-period" for BYU football either, growing up with Detmer and nationally ranked teams every year, so they were pretty smug as well, up until Crowton. Those that stuck around and stayed fans through the terrible years came out of the Crowton years much more tolerable and less smug. They have been much easier BYU fans to share in BYU's successes with, where had they never had to go through a crappy period, they would still be just as smug.

(Now onto a tangent) That, I think, is a reason why the new-generation of Utah fans (Those that joined since 2004ish) seem to be really high on themselves. But with the last couple years, and with (Let's be honest) the next couple of years, I think you'll see Utah fans become more tolerable because there really is nothing to be smug about.