There is absolutely no indication that TO would have sustained nothing but 9+ wins between 1997 and 2015. Joe Paterno was thought of as another legendary coach while at PSU and later in his years he went through some years that, quite frankly, the fans of PSU really thought it was time for a change......but......you can't just fire just about the only coach PSU has ever had.
In 1986, PSU had won two NCs in 5 years. They were riding high. Two years later they won 5 games and didn't make it to a bowl. Late in his career, he went through a 5 season period where he had losing records in 4 of this years.
Bobby Bowden was another legendary coach. Once he got FSU rolling in the mid 80s, they went on a spurt of 14 years where they never won fewer than 10 games. They won 12 out of 14 conference championships and two NCs.
Late in his career the program fell off from what it once was. He had losing seasons and ultimately was asked to retire against his wishes. I believe he was 80 years old when he was finally asked to retire. TO is 78 now.
Sorry, I love TO as much as any Husker fan. But, sometimes coaches stay too long. Sure, TO could have stayed longer than he did. However, at some point, it would have been time for him to retire and move on with life.