10 wins, which Bo has gotten with regularity, is not easy. Bo's accomplishments here really have not been bad at all.
That said, there are a boatload of teams that will have accomplished more in the past four seasons than Bo Pelini at Nebraska. These weren't Bo's first 4 years in the program, and he doesn't have higher accomplishments to rest on before that ('09 was a very solid season, though). We're not at a point where you could say this is an elite program hitting the inevitable downcycle. Rather, it's a program that has plateaued with nothing but hope that it will go break through to the next level.
I think there are some very good reasons to be hopeful myself, but that's just the nature of being a fan; eternal optimism. Also, most of those reasons (talent) are going to be sticking around even if the coaches don't.