Routes to MIA and BOS with AA will have a better prospect of lasting when BA and AA gain anti-trust immunity. As it stands BA couldn't be bothered codesharing MIA lest the premium passengers that it does carry MAN-LHR-MIA decided that MAN-MIA non-stop would be better.
Agreed when it comes to MIA, there were frequently more passengers in Business than Economy on the direct flight.
As for BOS I'm not so sure, it carried a BA code so that has little to do with it, although obviously it was an economy only flight. That said, it was very well supported by AA elites (try getting a First seat or even an exit row on a Friday morning, wasn't easy) and always went out near full the three times I used it.