This is more of a victim to competition for connecting traffic than anything else. Since LH bought out SN Brussels and Swiss, AA has probably lost a lot of market share at BRU and ZRH, which are both relatively small local markets.
Plus, UA is also in the ORD-BRU market with mileage reciprocity for the LX and SN loyal fliers who live there. There are better connecting opportunities beyond ZRH/BRU from the oneworld hubs in LHR and MAD for those customers who want the miles, so why waste the aircraft, crew time and fuel?
The only other routes out of ORD that could face a similar fate are FRA and CDG. Both have a large enough local market that I wouldn't expect either to go away anytime soon.