The Bus is where the growth in the domestic flying is (compared to the 737). Yes, we're getting a few used 737s, but not much compared to the 321 deliveries. The 737 Max is far away - 2025, and will likely replace the 737-800s I'm guessing in numbers, so little or no net growth.

Also, the 321neo are taking over long routes that used to be on the 737, so the 737 flying is trending to shorter, less efficient flying. (meaning, 2-3 legs a day instead of 1-2).