Here are some data I researched with regards to how coaches with particular backgrounds fare when they replaced a coaching legend over the past 30 or so years. For the purposes of this, I designated a "legend" as someone who had a great deal of success coaching at a university relative, either in terms of national championships or measured against historical success. I chose this because replacing a legend is a different process than replacing non-legend. In many cases, there is a desire to maintain the status quo, and hire someone with familiarity with the program. It can also be difficult to get an established, successful head coach, because replacing a legend is a hard thing to do. I think we can all agree that replacing Tressel will indeed be a case of "replacing a legend".
For each entry, the "legend" is listed, then the replacement, the replacement's background, and his success (or failure, as the case may be). I imagine this isn't an inclusive list, and certainly doesn't include all the hires made after unsuccessful coaches (e.g., Jim Tressel after John Cooper-- relative to historical success), and it doesn't take into account the myriad of reasons behind a particular success or failure, but it's a starting place. Take from it what you will.
My take on it is that promoting an assistant has a very low success rate, although bringing in a former assistant that was away from the school for at least a few years has a much better success rate. Hiring replacements that have no prior experience at the school, and were head coaches previously, also has a better success rate.
Assistants promoted from the staff of the coach that is leaving
Nevada: Chris Ault (Jeff Horton was an Ault assistant, although he was at UNLV as an assistant coach for one year before taking over for Ault in 1994. He left left for the UNLV head job a year later, and was fired after four years. After Ault?s second retirement in 1996, Jeff Tisdel was named head coach. He was also an Ault assistant, and was fired after four years.)
Colorado: Bill McCartney (Rick Neuheisel was McCartney?s QB coach, and was fired after four years.)
Oklahoma: Barry Switzer (Gary Gibbs was Switzer?s defensive coordinator, and was fired after five years.)
Michigan: Bo Schembechler (Gary Moeller was Schembechler?s offensive coordinator-- and defensive coordinator before that. Previously he was the HC at Illinois. He was fired after five seasons.)
Washington: Don James (Jim Lambright was James? defensive coordinator, and was fired after six years.)
Miami: Butch Davis (Larry Coker was Butch Davis?s offensive coordinator, and was fired after six years.)
Nebraska: Tom Osborne (Frank Solich was Osborne?s offensive coordinator, and was fired after six seasons)
UCLA: Terry Donohue (Bob Toledo was Donohue?s offensive coordinator, and was fired after seven years)
Georgia: Vince Dooley (Ray Goff was Dooley?s running backs coach, and was fired after seven years.)
Syracuse: Dick MacPherson (Paul Pasqualoni was MacPherson?s defensive coordinator, and was fired after eight years.)
Texas: Darrell Royal (Fred Akers was Royal?s offensive coordinator, prior to two years as head coach at Wyoming. He was fired after ten years)
Tennessee: Johnny Majors (Phil Fulmer was Major?s offensive coordinator, and lasted seventeen years before getting fired.)
Florida State: Bobby Bowden (Jimbo Fisher was Bobby Bowden?s offensive coordinator, and has served as head coach for one year thus far.)
Wisconsin: Barry Alvarez (Bret Bielema was Alvarez?s defensive coordinator, and has served as head coach for the past five years.)
Assistants one year removed from from the staff of the coach that is leaving
USC: John McKay (John Robinson was McKay?s offensive coordinator prior to one year as NFL assistant, and stepped down after seven years, then left USC for NFL HC job.)
Pittsburgh: Johnny Majors (Jackie Sherrill was a Majors assistant prior to one year as Washington State?s head coach. He left Pittsburgh for Texas A&M after 5 years.)
Former assistant at the school, but who had several years of experience as an assistant away from the school before returning as head coach
Florida: Steve Spurrier (Ron Zook was once a Spurrier assistant, but worked in the NFL for six years before becoming Florida?s HC. He was fired after three years.)
Miami: Dennis Erickson (Butch Davis was a DL coach for Miami under Jimmy Johnson before spending six years in the NFL. He left for the NFL after six seasons.)
Iowa: Hayden Fry (Kirk Ferentz was the OL coach under Fry, but was the HC at Maine and an assistant in the NFL for ten years after leaving Iowa. He is now in his twelfth season as Iowa?s head coach.)
Former assistant at the school, but who had several years of experience as a head coach at another school before returning as head coach
Ohio State: Woody Hayes (Earle Bruce was once a Hayes assistant, but was a head coach for seven years before hired as OSU?s HC at Tampa and Iowa State. He was fired after 9 years.)
Alumnus, but never coached at the school
Alabama: Bear Bryant (Ray Perkins was a former star player at Alabama, but had never coached there. Previous to being named HC at Alabama, he was the head coach of the NY Giants. He was fired after four years.)
Indiana: Bill Mallory (Cam Cameron played football at Indiana, and before that coached in college and the pros, albeit never as a coordinator. He was fired after five years.)
Virginia: George Welsh (Al Groh was a former Virginia football player and, briefly, assistant coach. He spent time in both college and the NFL as an assistant and head coach. He was fired after nine years.)
Both an alumnus and former assistant coach
Air Force: Fisher DeBerry (Troy Calhoun was both a former Air Force player and coach, although he spent the twelve years before hired as the Air Force HC as an assistant in college and the NFL. He is in his fourth year coaching Air Force.)
No connection to the school, and was never previously a head coach
Kansas State: Bill Snyder (Ron Prince was the OC at Virginia before being hired at KSU. He was fired after three years.)
BYU: LaVell Edwards (Gary Crowton was the Chicago Bears? offensive coordinator. He was fired after four years.)
No connection to the school, but with previous head coaching experience
Arizona State: Frank Kush (Frank Owens was Kush?s defensive coordinator and took over for Kush when he was fired midway through the season. Owens was not retained for the following year, and Michigan State head coach Darryl Rogers was hired, where he left after 5 years to go to the NFL.)
Miami: Howard Schnellenberger (Jimmy Johnson was head coach at Oklahoma State, and left Miami after five years for the NFL.)
Miami: Jimmy Johnson (Dennis Erickson was head coach at Washington State before being hired at Miami. He left for the NFL after 5 years.)