You originally implied that AIRLINES didn't want to pay for a new narrow body. They do. The C series/A-220 for one. Boeing is the one who doesn't want to spend the money on designing and certifying a new airplane. Boeing outsourced so much of its operation that it can no longer efficiently develop a new airplane. New airplanes always take a long time to reach an ROI and the board for Boeing doesn't have the stomach to wait a decade before they can see a profit. Much better to kick the can down the road and let future investors deal with the problem.

People love to blame Southwest for the current 737. That may be true for the classics and even the NG to an extent, but Southwest just told Boeing what they wanted to hear. By the time the Max was announced, the common type rating meant nothing. That saved airlines money back when FOs didn't get full type ratings and some airlines had pilots pay for their own training at privately run training centers. Now a new hire at an airline is going to receive dozens of hours of training so the additional difference in training is insignificant.