I've flown UA on this route. I have status on UA, so I was in E+, but for a 45-minute flight even E- is preferable to most seats on an ERJ, IMO.

Fares are low because WN flies CLE-MDW. AA was offering $40 (!) fares this summer.

I would suspect that UA and CO will codeshare on this route, given that it will offer easy connections at either end.

I think that, in the days when UA had a small hub at CLE, they ran even bigger aircraft than the Airbuses between CLE and ORD.