BYU will lose at least one game this year - and as soon as that loss comes, the enthusiasm of the fan base will die. BYU won't have a special seasons this year - they'll be lucky to get to 9 wins.
We should remember that the Nebraska game is not a foregone conclusion. Nebraska is a historically good (P5) team, with an awesome homefield advantage. Their coach is a good (though not great) coach. Taysom and Jamaal will both be playing their first game back from injuries, and BYU will likely be dealing with suspensions as a result of the bowl game fight.
Is it winnable? Sure - even FBS teams knock off big teams every once in a while. But make no mistake, BYU is the underdog in that game.