The GOP won the state-legislative coinflip after 1990 and, dutifully, drew a map favorable to the GOP. The Democrats appealed the the Supreme Court who ordered a number of districts redrawn. Some of those districts were struck down as "non-compact." A significant number of the state's current legislative seats are less compact that the ones struck down in the 90's. If the GOP gains control of the IL Supreme Court they need only follow precedent to strike them down.
Thankfully the Democrats could redraw the State Supreme Court districts before the 2022 election.
Issue though is counties can't be split and Cook is the first 3 districts as mandated by the state constitution which would require an amendment to change. You can still draw 2 Collar county districts but at the judicial level the collars are still relatively speaking swingy. It would improve their chances still but I wouldn't call it a slamdunk. There is a decent excuse to redistrict as the districts have pretty varying population for the non Cook districts.
To be fair, even with a "counties can't be split" requirement gerrymandering the IL Supreme Court is very easy. Here is how a hypothetical gerrymander would look like (the 4 downstate districts have populations of 2.1, 1.9, 2.0 and 1.5 million; which is way closer than they are now but I'll admit, not perfectly equal).
District 1 is the Cook district and obviously you can't change that and it's safe D.
District 2 is meant to be the district that puts Dems over the top (Clinton+11, albeit only D+2)
District 3 is meant to give Dems another possibility of attack and a 5th seat (Clinton+6, D+1)
District 4 is a Republican sink
District 5 is Likely R, but maybe could flip in a wave (Trump+8, R+5)
Alternatively if Dems want to have a "clean" map so they can't be accused of gerrymandering that is also easy to do and has a similar effect (other than the District 5 equivalent which becomes Safe R)
