I don't think any of the major cities of the Midwest are at any risk of losing HQ's to Chicago. They all have adequate air service and connectivity. It doesnt match O'Hare but it is adequate and if the C-Suite is content then they wont up and move.
The most vulnerable are the 3rd tier cities like Battle Creek, Ft. Wayne, Benton Harbor, Evansville,
Take this company for example
Franklin Electric Grew from a rural machine shop into a true global company through acquisitions and good management. However now they have 23+ locations on every continent. And a manufacturing plant in Madison, WI and Ft. Wayne IN. How much longer will the C-Suite operate out of Ft. Wayne. Right now the Directors and C-Suite is populated with experienced professionals from other major companies and many have MBA's from Northwestern and Booth. This is the profile of a likely corporate relo while leaving all manufacturing in place.