Here would be my defense of my methodology:
1. First and foremost, this was easily compilable using Wikipedia. I have neither the time nor the math skills to make some intricate five-factor regression that would tell us essentially the same thing.
2. Like I said, this is about us moreso than the Illini. I actually got the inspiration for this on a treadmill at my gym a few Saturdays ago as I saw people in Wisconsin, Michigan State, Iowa, etc t-shirts walk by me. And I was thinking about how the measure of success in a fan's life is really being able to hold victories over your rivals. You could express the Illini's failure in a lot of ways, but the most emotionally resonant one is that we never have the upper hand on our peer institutions whose fans surround us every day. That's where the above .500 conference record came from. But then it also seemed like it was hard to define non-bowl/tournament seasons as successes, which also had the benefit of eliminating seasons where teams are in NCAA jail, which seemed like they shouldn't count because they don't feel like they "matter".
"Mattering". That's what I'm driving at. Sports is fun when your team matters. It sucks when they don't.
(EDIT: Another reason I didn't think of. Doing it any other way would throw the balance between Football/Basketball out of whack.)
Nice work.
Underneath all of this is the fact that we haven't had good coaching quality for years. Tepper, Turner, Zook, Beckman - where are these guys working now? Weber is middling along at KSU and Groce is on a very hot seat.