They are not going to buy more NGs. The NG is technically still in production but production has wound down significantly. All orders for airlines have been bui

They are not going to buy more NGs. The NG is technically still in production but production has wound down significantly. All orders for airlines have been built, and all that remains are orders for the military version and various private customers. It would probably cost somewhat more for Southwest to order NGs now than it did during the production heyday where economies of scale made the plane cheaper to produce. Buying brand-new, less-efficient 737NG planes when competing airlines gradually have more and more A320neos could allow Southwest to keep growing, but might not be a great long term investment. Southwest can keep flying their existing NG fleet for a very long time, however, so they can (and have to) wait for new planes. Southwest isn’t forced to stay with the 737. It’s all about costs. Buying new NGs probably would cost more in the long run than (years from now when the backlong is reduced) starting to operate the A320neo, but far cheaper than either is if the MAX returns to service and they get hefty compensation from Boeing.