There is not really a question of whether he would have continued to improve in future years. Riley Nelson broke his back in the Weber State game and went on to put BYU fans through the most infuriating season as far back as I can remember. Are you saying BYU would not have been better off with Jake Heaps as an option two years ago.
In that case, Hill would have been able to redshirt that year and would have 3 more years of eligibility.
If we had a decent QB the year that Ziggy and KVN went off, we could have been one of the best teams in the country.