For one thing I will admit that, unfortunately, there are some fans who are Utah fans due to their hatred of BYU (this is a very, very small subset of our fanbase). Although, playing BYU won't stop them from cheering for Utah.
If Utah does well, even when we don't play BYU, they will use that to rub in the faces of BYU fans, etc.
They may actually like college football in general as well. Playing teams like USC, UCLA, Oregon, Stanford, Washington, etc. every season, will still be a nice thing for them.
Again. I think it is a gross overstatement and a BYU fan pipe dream that BYU means that much to Utah.
How about the rational BYU fans that will also cheer for, spend $$ with and otherwise become secondary fans of Utah's due to our schedule, conf. affiliation, etc.
There are many Mormon fans in the PAC footprint, who are adopting Utah as their team in the PAC due to state and religious affiliation. Yes, those folks are largely BYU fans. It's happening now. I actually think Utah picks up way more than it may lose.
Also, losing the type of fan who only is a fan due to BYU hate, is a good thing in my opinion.