"Getting the cars back" usually implies waiting for the flight IROPS to cease and the normal one-way traffic to return to normal. Then one-ways from Chicago and other cities to Detroit would eventually replenish the inventory count, presumably with different cars.

The problem here is that it's not a snowstorm, where there's an abnormally high number of random one-ways in all directions, both inbound and outbound, which would roughly cancel out long-term inventory fluctuations. The one-ways in question were all in the same direction, and it's unlikely there will be a huge influx of customers wanting to go to Detroit in the next week. One-way rates into DTW from other cities with extra cars may become lower than normal, for those who are heading there.