In my opinion, the perception that nobody wants to be the guy that replaces a legend is very over blown. Sure, when Coach Bryant retired and Perkins was brought in, that was a tall task, but it had to be a Bryant guy so the pool was small. In today's game, you aren't bringing in the best Alabama guy you can find, you're bringing in the best college football coach you can find. It might just so happen that the best we can get probably has Saban ties.
Dennis Erickson replaced Jimmy Johnson who replaced Schnellenberger. Day replaced Myer who replaced Tressel. Not that any of those guys are Saban, but success can be maintained. When Bowden and Paterno finally stepped down, the programs were in shambles. The closest I can compare the state of the program Saban is leaving is when Stoops stepped away from Oklahoma, and Riley was able to maintain success.