Hill would have redshirted his freshman year (like he should have anyway) then backed up Heaps during his Redshirt freshman year. If that was the case, he'd have 3 years of eligibility left right now.
If you do recall... Jake Heaps came in at the end of the year in 2011 and looked pretty dang good. A lot of coaches would have left Riley on the bench because Jake was playing just as well, if not better, than Riley was before the injury.
It's quite presumptuous of you to say that Hill would not have come to BYU had Heaps stayed. Do you really have any inside info on this topic or are you just making assumptions (you know what assumptions do to you, don't you?)?