I'm sorry to break it to you but it hasn't been that great. It's been good no doubt, but you're equally as jaded by the Groce years. If you want to look at 4 year spans, Underwood has won 88 games these past 4 years. Groce's best 4 years was 77 wins. Weber's best was 112. Sure you can say but Weber inherited a monster. Ok that is true. If you start after the title game, the next 4 years, Weber won 89 games. And that included a really bad 16 win season. Underwood's best season is 24 wins. Weber did that or better 4 times. Groce's top was 23 wins.
So if you look at Illinois basketball over the past 30ish years, Lou had us rolling and satan killed it. Lon Kruger brought us back, Bill Self took us to the summit (or at least close), Weber continued the climb to the peak, but his poor recruiting brought us back down to just a good team. Groce was the flop. He couldn't get us any better than good. In the last 20 years, which encompasses all of Weber, Groce, and Underwood, the 2 worst seasons belong to Underwood.
These are just the facts. I think Underwood is certainly a very good coach, and there is no debating that. However, I also don't think he is someone like Self where you feel deflated if he leaves. I personally don't think he is a top 10 coach in the nation, despite being paid like one. Ultimately I think Underwood is here next year, but if he's not, I'm personally not too concerned.