I would say that in a BCS conference, real success is defined by winning NT games, making NT runs, etc. Being a consistent PI team with the occasional NT berth isn't successful, it's just consistently mediocre.
Let's be real here: There are roughly 150 human-coached DI teams in Allen, so nearly 2/3 of human-coached DI teams will make the postseason. To me, that means just making the postseason isn't a success -- particularly from a BCS conference where you have an inherent advantages over more than half the teams out there.
And swamp, that's not meant as a dig at you or anyone else. But I do think that calling your tenure at Purdue successful would have to be the result of very low expectations. (And on that note, it's also worth pointing out that you took over a program that was in the NT nearly every season, so their level of play has clearly sunk w. you at the helm -- and that's w. the Big Ten not being as strong as it used to be.)