If reps are hard to come by, I understand his decision. Bodies for next year on the interior are going to be high - Keaanaina returning from injury, and Rubio's emergence. Add in Onye, Ford (if he doesn't move outside), and with Howard Cross returning as a graduate.
I'd love to see Mills' snap count, showing what percentage of his overall snaps came playing on the inside. However, he does seem more effective overall as a inside player. Maybe that could be in the future plans - him playing inside in a four man front.
There was still a role for Lacey on this team, but perhaps it just would have been limited. And I can understand him wishing to save a year of eligibility. He opens a spot for younger players and he can explore new opportunities for playing time and potentially reach the NFL.
Just have to say, Lacey fought off a lot of adversity with injuries the past two seasons, as well as the health of a close family remember. His determination and strength will lead him somewhere great in his life.