The last 5-6 years of Sabans career he lost his ability to hire top quality assistants. It showed on offense and defense.
I am not sure it was so much his inability to hire so much as he got completely gutted. It's not that unusual for your coordinator to take an HC job, it is somewhat unusual for a coordinator to leave and take a job like Texas or Georgia and in both cases they left with key offensive and defensive staff members.
So, for instance after Saban lost Pruitt, it wasn't just that he'd lost Smart, and Smart's replacement, but he also lost the replacement's replacement. Mel Tucker, Lanning, Schumann, all guys that were under Saban. When Sark left there was a similar drain, notably Kyle Flood and two other coaches.
That was an incredibly unusual situation, you don't usually lose chunks of your staff like that and I think he never fully recovered. I'd also note that the three coaches Sark took with him are still at Texas and appear to be a vital part of his success, and the guys Kirby took with him had so much success two ended up head coaches and another is current DC.
One could debate though if he became attached to some guys purely because they stuck around, valuing continuity over excellence (though to put it another way it would be like trying to grow another crop after your last one got destroyed), but the sheer amount of coaches he had to replace is amazing. Some coaches spent the better part of their lengthy careers with the same coordinators.