I really don't think Mack and Miller were going to Illinois. Miller got the IU job and was probably waiting for a couple spots to open up. Same with Mack (who is an Xavier alum, by the way, and has a good thing going there). (And it's not like Miller was dominating a power conference or put up crazy efficiency numbers like Marshall. He never finished above 39th in kenpom at Dayton).
So you're left with Davis and Bennett, two guys who are older than Underwood and have been at their jobs for fifteen+ years, and who aren't demonstrably better that maybe you could've gotten for cheaper.
One final thing to consider -- when paying a coach, there are certain incentives to paying top dollar -- you want recruits to think you got a hot name, a top coach, etc. Paying Underwood more serves these purposes. If they'd hired Randy Bennett for $1.5 million, it probably wasn't going to impress recruits or people in Midwest basketball circles.
In the end, this seems to me like it's more about Underwood screwing OkSt and getting rewarded than that there were other obvious great options.