Well, it's not a terrible idea. I just don't think it needs to be an interstate. Lay out a 4-lane divided highway with intersections at-grade and bypasses of major towns. This is what Ohio and Indiana did to US-24 between Ft. Wayne and Toledo or US-30 between Valpo and Ft. Wayne.
There's also gotta be some cost control. Ohio built its Fort to Port project (the segment between between Napoleon and Waterville) for
$8M/mile, to expressway standards, on a greenfield alignment. Illiana is projected to cost at least $27M/mile, even though there are virtually no differences when it comes to terrain, and the economies of scale should make it cheaper than the Ohio project.
The way I see it, with construction costs so absurdly high and no movement to bring them down (higher costs = more jobs, right?) we need to be building only the highest priority things, and Illiana is not that important.