It's surely a matter of management (small 'm') Airlines codeshare and run close to capacity because the competitive market sets prices that need this to break even. Crew will need to DH for positioning - this is predictable. Any delay downroute can take crews beyond permitted duty hours at short notice, so there needs to be contingency planning.
In the past this was done by DHing at short notice and there was usually the spare capacity to accomodate it. But the airline needs a decent contingency plan for when there are no unsold seats available. I would suggest bumping passengers is a high-risk answer. I already have three airlines on my "not ever" list* and I'm thinking that UA is probably going to make it four for this reason. So what's the alternative?
Well a very brief chat with Mr Google reveals the large number of smaller GA airports around Chicago which have a lot of short-term charter companies available (and I'm only looking at anthying King Air or above). Were I the risk manager at an airline I'd be looking to have a call-off contract with many of these by which I could get needed crews flown from the smaller airport to a GA airport near the required destination at short notice. This would also provide me with an actual cost to feed into the risk calculations to trade agains the costs of keeping spare capacity and/or bribing passengers to accept bumping.
* Delta, AA and Ryan