I don't know how anyone can end up stranded at ORD, knowing this far in advance of all the cancellations.

And I'm assuming UA can't just add routes at will, so a flight that would go IAD-ORD-DEN can just go IAD-DEN, skipping ORD for a day or two. They may as well just keep the planes at ORD so they are ready to go when conditions improve. Plus non-hubs may not be able to "store" planes at all. If they do move planes out I would guess it would be to DEN or SFO.