worse schedules. Those teams might play 1 P5 on their schedule. It's not likely that everyone BYU beats is just absolutely awful. Their schedule will look better than Boise, Cincinnati, USU, Fresno, Army, etc no matter how you look at it. I personally don't see a point in talking about what is technically "possible." What's likely with 10 wins and this schedule is some respect for one of the toughest starts in the country, and then a ranking 15-20. Or I guess I should say 15-19.
The only double digit win teams who are stuck that low are G5's who play nobody. The 10-2 you saw from Cincinnati with only an awful UCLA team as their P5, ends up top 25 with a VT win to make it 11 because of the former part. The rest of the schedule doesn't cut it.
I agree with you that IF BYU only ended up in the mid 20's with 10 wins, it would have to be because everyone totally sucked. But I don't think that is the likely scenario. The problem here is with the OP's suggestion. I know it's a what-if and a "what would you take", but it should be more realistic.