HS students aren’t leaving Illinois because of UIUC. They are leaving because NIU, SIUC, NE Illinois, and Western Illinois are failing. Most are not potential UIUC students going to UW Madison or Purdue, or even UMN or Iowa, they are students who would have attended the smaller public universities in Illinois, and are attending their counterparts in other Midwestern states.
BTW, UIC is booming. They cannot keep up with the demand, and their CoE is adding new buildings, hiring new faculty like crazy, etc. However, UIC isn’t a good option for students from downstate - it is farther away than many OOS colleges, and, if you don’t already live there, Chicago is expensive, and there are still too few dorms.
Other public colleges which are doing well are Illinois State (Bloomington-Normal), while Eastern Illinois (a very rural university) has turned their trend around, and their enrollment has grown for the past two years, and has done so substantially.
UIUC has always has a very strong bias in application - despite having a 60% acceptance rate, the stats of their students are similar to UMN’s even though UMN has a 45% acceptance rate. Illinois has always had, in the past, a very large set of good public universities aside from its flagship. SIU and NIU are research universities, and most of the rest are either comprehensive universities or very good master and undergrad universities. As most of those deteriorated, their students went elsewhere. While UIC absorbed many of the Chicagoland students who would have attended NIU or SIU, downstate students or students who would have attended WIU, NEIU, etc, find other colleges out of state.
There aren’t a slue of new kids with 4.0 GPAs and 1510 SATs who suddenly find UIUC to be too pricey. The huge increase in tuition at UIUC happened between 1999 and 2007, and hasn’t increased much since. Tuition at UMN isn’t much lower than at UIUC, BTW, and tuition at UW is higher. Even with merit money, places like UMN or UW still will cost the same as UIUC in-state.
On the other hand, a high stats student who cannot afford UIUC or schools like UMN with merit, can get much better aid at Alabama or Kent State than they can at SIU or NIU, and these schools are overall better.