he hasn't dipped below 8-5. BYU will not fire him unless there are off field issues.
my main point is that even under the situation where it is possible BYU would fire him based on winning percentage, it wouldn't be wise. 8-5 should be good enough to fulfill your current contract. maybe not good enough to get a new contract, but it's not pink slip worthy.
to do otherwise would place the university in a position of trying to allure a potential head coach to a job with financial, recruiting and other tactical disadvantages, while placing expectations on him that are reserved for upper tier P5 programs.
We wouldn't get the head coach we want if we fire him.