Without reading all the above, the 737-800 has been kicking the tail off the 320ceo forever. It has more range and better seat economics. The 320 will always be more spacious than the 737 internally, but the economics haven’t been better than the NG until the NEO came onto the scene.
The higher height under the wing of the 320 was better situated for the new generation of big fan engines. Even with those, the 320neo isn’t great, although the 321NEO clearly is. Boeing, in response, went cheap and is stuffing big fans under the 737 wings, which required a major slung forward pylon change (trailing link gear for the -10) and this ridiculous MCAS system which was obviously under engineered.
Cost cutting obviously has caught up with Boeing. It’s sad because all the signs were there during the 787 and 747-8 programs. It’s sad that greed has now cost lives. Perhaps they will learn for 777X, NMA, NSA...