I think the defense will be fine and will continue to be near the top in the MWC. Lose two good LBs but redshirted a very promising junior to take Helmuth's place. Mosby, a really good safety, got moved to LB and started a few games over our returning starters at LB based on the matchup. Kelly is a big loss; I look to Wylan Free or Chris Gaston, two freshmen that got a lot of PT, to take over for him.
The DL (as it was for most of the year) returns by and large.
FS loses a lot at WR, like Boise. The remaining guys seem to be very athletic but you don't lose polished veterans and expect things to stay the same. Losing McMariyon will hurt too, but Reyna is more than capable. The OL has been patchwork all year.
Like I said, Boise will be the favorite, but I think it'd be far-fetched to say that it'd be better than this year. Hawaii, SDSU and Nevada could take FS out, even though FS more or less handled each of those teams this year. The same could be true for Boise. I'm guessing few Boise fans predicted a loss to SDSU at home at the beginning of the year.
I should also add that I think that the question of how "good" a team is doesn't have a 1:1 correspondence with outcomes because of factors like schedule. You rightly pointed to the inter-divisional rotation and I think that makes a difference.