As a Platinum who does EWR-ORD 2 or 3 times a month, on discounted fares--I've never not been upgraded
I usually fly mid afternoon (the 1:45 or 4 pm) out of EWR, so that may be why there is availablilty. As for MDW--they are now flying 737's again, but my flight yesterday (1:25 out of EWR to MDW where I again was in FC) was so full, they asked for volunteers--with the choice of 200 credit Plus a FC seat on the ORD flight or FC a later MDW flight.