Manning was atrocious the latter half of the 2015 regular season. From the home game against Minnesota until week 17 against San Diego he threw 4 TD/14 INTs. Osweiler fared better but still wasn't deserving of that big contract in Houston.
But I generally disagree with the premise that you don't need a solid QB to win a Super Bowl - besides a few outliers there is little evidence to support this assertion.
I thought Manning was injured for the latter part of that year, which is how Osweiller got to play...led them to the playoffs at which point Peyton took over again.