As suggested above, the people who say Illinois is a top 10 job are talking about potential. It's obviously not a top 10 program right now, but it has everything needed to be one on a consistent basis.
In terms of potential, Illinois is one of the top three basketball jobs in the Big Ten. It probably has more talent than any Big Ten state, and it has only one state school competing for that talent. It is THE Univerity of Illinois. Take Izzo out of MSU and what have you got? A program that excelled in the first instance thank to the fortuity of Earvin Johnson growing up there, and then Izzo rasiing it up himself. Michigan has a good history but Illinois has better potential. Indiana and Ohio State are the only two that rival or exceed the potential of Illinois. If Illinois ever found a coach who could consistently land the best home-grown talent, and actually coach them up, they'd be winning conference titles and making deep NCAA runs on a regular basis. And the coach would be swimming in cash.
That's why Illinois is a top 10 job in terms of potential.
As an aside, there are some jobs that most people would list as top 10 programs right now that may not remain consistent top 10 programs after their current HC's retire (or are run off). That list could include Duke, Syracuse and Connecticut, for starters. It's very difficult to follow a legend, and really what is there about any of those schools that makes them preferable to any number of other good programs equally close to the big recruiting hubs? They could all fall off the list a coach or two down the road.