Would he have been? There have been many QB's that have been ruined because they were thrown to the wolves too soon. Not every QB is ready to start from day one, nor do they have a great grasp of the offense.
The fact is the Chiefs traded up to number 10 and gave up significant capital to acquire Mahomes, then they proceeded to give Alex Smith the nod as starter. This is despite the fact that the Chiefs were an 11 win team. Why do you think that they started Smith over Mahomes?
The answer can only be one thing, Reid didn't think he was ready yet. That one year can go a long way. The first benefit is having a greater mastery of the offense. Coming in year two if you did your homework, you have a good grasp of the playbook and all of the moving parts that are needed to make things work. In addition to that the QB has a whole year of experience preparing for opponents, film study, and experience observing what a veteran needs to do to stay relevant in the NFL. Let's not forget an additional year of developing chemistry with the wide receivers and TE's.
The NFL game is very different from the collegiate game. There are things that you can get away with in college that you can't at the NFL level. The speed is different and your throwing windows are much tighter and shorter. In the collegiate game, those passing windows are doors in many cases. The defenses and offensive structure and even the rules/fields are different.
Mahomes was not destined to be great no matter what he did, not even the NFL thought that. It was a combination of being brought up underneath one of, if not the greatest offensive minds in the NFL, a stable situation and getting the time needed to develop that turned Mahomes into Mahomes.