AA and UA don't have the extreme overcrowding issues that DL has. Thanks in part to separate premium class lounges (FL and Polaris Lounge), but also not having Amex Platinum access. Never say never, but I don't see AA nor UA needing to do this, it's not a cost saving measure, just dealing with the overcrowding they have. Actually I'd say they'll lose money with restrictions on buying club memberships since those lower elite level flyers who buy a membership are the ones using the clubs the least. Even for DL it may even be a temporary measure until they get their dedicated business class lounges.