Let me explain how building quality depth works, and how it applies specifically to bringing in a stud prospect like Hanna despite having a "loaded" secondary.
Cisco was a freshman last year; this year, he will be a sophomore. Meanwhile, Hanna will be a senior in high school. When he arrives next year, Cisco will be a junior. Ideally, Hanna redshirts while he learns the system, coinciding with Cisco's junior year, to acclimate to the pace / speed of the college game, familiarize himself with the playbook for his position, and spend a year in the S&C program to get ready for college ball.
By the time he completes this redshirt season, his first year of eligibility will be Cisco's SENIOR year. During that season, Hanna will then hopefully carve out a role on special teams, or perhaps on the two-deep where he can earn some developmental PT, to gain some experience to prepare himself for playing a more figural role the following season when Cisco is gone.
And - best of all - instead of being rushed out onto the field before he is ready and / or it is physically ideal for him to be on the field, the team has the luxury of getting him ready first, to maximize his long-term strategic value.
That's why you take a safety with Hanna's potential this year, in lieu of there being a hole in the depth chart. The perfect scenario is to build quality depth, develop said depth, and let players ascend as a natural progression of their developmental curve.