15% (if they hit that) is going to put it about 5% behind the C series on a CASM.

Boeing's reaction is improvements to the 737 that have already netted about 7% and the realization they can't stuff a bigger fan under the wing of an airplane with little landing gear. Bigger gear means new wing and huge costs.

Meanwhile, Boeing's got a big order book for the 737. As long as it sells, they will milk the cash cow. Development costs on the 737 were amortized eons ago.