Regarding the OP: First of all a bit of an apples/oranges comparison, different types of Qs. But, a good point nonetheless. There are certain elements of football that aren't Rocket Science unless you keep trying to make it that way.
Funny thing about successful college athletes at FSU, Miami, Bama, wherever; they usually improve greatly with EXPERIENCE and playing time. Case in point: Remember fans were complaining about Berrios? He made some pretty significant improvements.
If an athlete who has worked his way into the starting lineup doesn't make improvements, and someone else has. then the starter gets replaced (or in the case of a freshman or transfer who comes in better than your starter, like Bobby Boucher did at University of Louisiana).
For all of last season, and at this point in the program, as much as we love to kick this dead horse, there is a 10-win QB and no one else that fits that replacement bill. Does this mean he's a Heisman candidate and we win a NC like Bobbie's old guy? No, but I bet he improves.
I know you wanna-be NC2A QB coaches aren't going to like this, but, spring showed that Rosier is still starting because that's who the guys who actually draw a pay check to make those decisions, think can best lead the team.