Allen and Trill were the only 2 scholarship QBs in 2014. Trill was a true soph. If it was just about getting beat out for the job, most coaches would have kept Trill around in 2015 as an insurance policy. Kyler is unproven, Trill is good when his head is in the game.
QB depth is critical, especially in a passing offense. KDS would have only cut Trill loose if he were a cancer.
Joekel/Showers/Davis lacked the talent to get it done. They were "in case of emergency break glass." Trill had the skills to play at a high level. He was/is just an immature dumb butt.