Originally Posted by
number_6
Earn-only means they pay USD to AA (and AA gives them miles). AA likes that. Redeeming requires AA to pay them USD (and they give AA a seat to use for award travel); obviously AA doesn't like that as much. In any case it is AA that decides -- and you see the decision. No reason to expect it to ever change, or to think that any of these airlines will be joining OW in the next decade. It is just a marketing scheme for these airlines (like advertising).
And yet, though in the opposite direction as you discuss, the relationship between AA and TN did change relatively recently, as we now can earn miles on TN instead of simply being able to redeem them.
Likewise, the relationship with Gulf Air changed from simply earning to now earning and redeeming.
While it may just be a marketing scheme, clearly things do change, and I don't find it improbably that an airline like TAM, MX, or someone else couldn't join OW in the next 5-10 years (if not sooner).