Actually, the addition of Aguilar--even if he wins the starting job--merely keeps Merk and GMac on schedule in their planned development curve.
They certainly expected Nico to start this season. And the plan Heupel sells is that we will recruit the best possible QB candidate each year, and that ideally, if you come to Tennessee as a QB, you will have 2-3 years to develop while competing with players ahead of and behind you to sharpen your skills while deep-learning the offense. Then, if you earn it, you can have two seasons to really shine in the offense, and establish your resume for the next level.
Inevitably, injuries and rogue dads will interrupt that sequence, but it's the best and fairest compromise between offering players opportunity to compete at their best, while protecting the program against very bad luck. IMHO