Having flown LAN a lot, and TAM a bit, I would (and often do) choose LAN any day of the week. This is independent of the fact that their being in OW is another significant plus for me.

If LAN drops out of oneworld, it seems to me it would be a major setback, at least in South America. There really aren't any other major partners OW could bring into the fold. I don't think LAN would benefit much from the loss of AA and IB as partners (both quite strong feeders into SA).

Quite frankly, a LA-JJ tie-up would also strike me as getting a bit anti-competitive... which makes me worry that both carriers' weaknesses would be likely to shine moving forward.