One more thing... UA already "allows" AA to operate routes on which it doesn't compete. Say, ORD-BRU. UA does IAD-BRU but leaves ORD-BRU to AA. Or, for so long, the ORD-Hawaii routes that weren't competitive.

Just because ORD is UA's biggest hub doesn't mean that UA will match all AA's routes.

On the flipside, AA pretty much owns South America-- yet doesn't match UA's ORD-South America flights.