"Utah's mediocre past has helped ground expectations. So a run like Utah has had since 2003 - far beyond even it's biggest fans' expectations - is met with euphoria. At the same time, the core of Utah fans still maintain a reasonable set of expectations."
That core of Utah fans you referred to obviously doesn't post on message boards. They certainly don't post on CB, UF.net, DN, SLT, etc. So the perception is that the typical Utah fan is the one posting vitriole against BYU.
There's a handful of reasonable ones on CB that you can have a healthy discourse about regarding both programs with points and counterpoints going back and forth. I agree it's really not any different with typical BYU fans and the reasonable ones seem to be in the minority there too.
The one problem with your analogy is that Utah is still coming down from their highwater mark of 2008. Whittingham gets a free pass still on major goofs and gaffs. Case in point, last year's MAJOR screw up with the QB depth and how it wrecked your season and absolutely blew a wide open shot at winning the South Division and a chance to play in the inaugural Pac12 Championship game. How HUGE would that have been for Utah in their FIRST season in the Pac12 to pull that off? It would have been Epic.
But, because Kyle kept switching his best QB recruits to other positions, he gambled that a fragile 165lb QB recovering from 2 major shoulder surgeries and a Div 2 QB would be enough to make it through a Pac12 schedule behind a shaky offensive line (2nd worst in the Pac12 for sacks given up).
Not only did Utah fans not hang that screw up on him but the Utah media was afraid to do it as well. Most attributed that gaff to "the coaches" and one Utah sports writer even tried to blame it on Norm Chow who'd been there less than a year. It's still too close to the Sugar Bowl win for him to suffer any ire from unhappy fans but that doesn't mean there won't be some to come.
Remember how quickly Utah fans wanted to hang him out to dry after Utah's 6th place finish in the MWC in 2005? Because Urban was gone and the Fiesta Bowl grace period went with him, Utah fans were calling for Kyle's dismissal after losing to Colorado St, New Mexico, UNLV, Wyoming, etc in 2005 and 2006.
BYU's highwater mark was the National Championship in 1984, the pinnacle of the sport. Nobody called for Lavell's head in the years after that even though they had some 7 and 8 win seasons but there's been plenty of time go by, enough for fans to start feeling some angst and a desire for getting a taste of that kind of success again. In all honesty Bronco may have made it worse on himself by talking about delivering a National Championship and then bringing in a QB who was the #1 QB recruit in his class that reiterated that proclamation.
If you make bold claims you better be ready to come close to the mark or there will be heck to pay. I think the one difference is Kyle doesn't have anything to live up to. Urban wasn't there long enough to establish a legacy, Ron McBride did a nice job but didn't do anything remarkable for Kyle to have to live up to. Kyle gets a free pass but the guy who comes after Kyle will have a lot to live up to or Ute fans will give him heck for not delivering. Kyle is smart enough to know not to say anything that will bring any of that kind of expectation on him. Bronco could learn from him.