People talk about the improvement of Zach Wilson or Steve Sarkisian....but those were already fairly accurate

Sarkisian completed 64.9% in his first year at BYU, before upping it to 68.8% in his second year

Wilson completed 65.9% as a true freshman before getting up to 73.5% his junior year (against the COVID schedule)

Retzlaff completed 50.4%

So the best BYU example is John Beck. He completed 50.3% his freshman year on a similar number of attempts as Retzlaff. By his senior year he was up to a 69.3% completion rate.

The caveat to the Beck success story is that it wasn't overnight or even one season to the next. His completion rate went 50.3% -> 56% -> 64.5% -> 69.3%

It really wasn't until his third year at BYU that he got an even adequate completion percentage. Retzlaff doesn't have that much time

So instead consider Joe Burrow. He went from not getting FBS snaps to starter on a new team where he was pretty mediocre. He (57.8%, 3.2 TD/INT, 7.9 AY/A, 133.2 rating) was definitely better than Retzlaff (50.4%, 1 TD/INT, 4.6 AY/A, 97.1 rating)....but he wasn't good and accuracy was a major concern

And then the next season he jumped to the best QB performance in college football history to that point and a 76.3% completion rate