Game Set and Match... AA won.
AA commands MIA through its sheer size and marketing relationships (e.g., TAM, LAN, TACA, Iberia, and now Mexicana).
There will be no coming back for UA.
Indeed, it would behoove UA to look carefully at how AA has locked up the market from the US to Latin America. It might learn something about how to protect its leading market position from the US to Japan and beyond.
In Asia, AA is the underdog, but nothing that UA has done really stands in the way of AA expanding there. The only impediments are full liberalization of the US/Japan and US/China air service markets. With a few more 777's, several 7E7's, additional slots at Narita, and route authorities to China, AA has ever opportunity to grow in Asia by bypassing traditional US/Asia hubs and picking off the most premium routes in the US/Asia market. And, so far, UA has done virtually nothing to preempt that point to point attack.