Sure, Sitake coached for a whole year at some podunk school in Arizona, and then 2 years at some podunk school in Utah. If that really props him up that much higher in your eyes, you appear easily impressed.
Neither coached at the FBS level before their current gigs. Neither has coached for more than one FBS HC. Neither has designed nor gone somewhere outside of their only HC's shadow to be in charge of his own defense. The only real difference between the two currently is Sitake is a few years ahead of Howell in coaching at the FBS level, something that would be completely meaningless by the time Bronco hypothetically retires.
I'm with you that Howell will need to prove himself outside of his only HC's shadow before I'd ever consider him a candidate for an HC job. I simply apply the same standard to Sitake.
One last thing about Howell: If you'd remove your bias, his resume is quite impressive for a young coach. It took him two years at the high school level to get an HC gig. That's not an unimpressive feat. In 4 years at the FBS level, he's rocketed up to DC, right past guys with seemingly more impressive resumes. Also not an unimpressive feat. We'll see how he does as a DC, but his rise through the ranks is impressive if you'd take an outsider's view.