I give SWA a lot of credit for making good financial decisions, but they are a 100% domestic carrier. Also, I would bet that Southwest carries a higher percentage of passengers who are price sensitive than Delta. Delta is not a domestic only carrier either - I forget the exact figure, but 40% or above of our traffic is international, which may not have the same hit to traffic.
On my flight to Moscow last month, only 3 passengers out the entire airplane were American looking. Now, I didn't speak to all 220+ pax, but it was clearly not an American crowd traveling.
Delta goes to a lot of places where their economies are doing just fine, thank you.