I'm trying to think through how this will affect demand (as manifested via load factors, fares, upgrade policies, etc.) on both BA and AA.
The obvious outcome is that at least some of the folks who currently fly AA to LHR will switch to BA. Flat beds in J are nice going east, after all, and I think most people would find BA's F product superior to AA's (with the possible exception of the Flagship Suite -- only available on some LHR routes, but not from my own BOS, for example). All this suggests that to keep at least J loads balanced, BA would need to charge higher fares, AA would need to offer more bonus miles on its own metal, or the market would need to equilibriate in some other way.
Has anyone else thought this through? Sketched out specific possible methods of assuring that loads and fares remain in balance? Estimated how big a factor this might be? I didn't find anything in a quick Google search or a quick search of this forum and BA's, but there may be threads or columns that lurk nonetheless.
My own guess: When buying J fares, I'd probably stick with AA and upgrade to F (which I don't expect to be able to do on BA, of course). If no AA F upgrade were available (for whatever reason), I'm pretty sure I'd choose BA over AA in transatlantic eastbound J, especially if LHR connections and lounges remain as they are now, but for perhaps 10000 extra miles each way, AA could keep my busines on their metal. (Not sure 10000 miles is really my price, though. Have to think about this some more!...)