Unless they can sell on the craft they own, or can offload lease structures they are locked into, There is sunk cost. They'd have to wipe out a huge amount of (possibly paper) money to walk off the 737NG fleet. I believe the Max purchase was fed from growth, as well as replacement, but in shrinkage, It would be untenable to acquire a paper loss, and have to spend real money. Yes, the operating cost is lower, and its a nicer plane. But, it also has certification issues, and will now demand (probably) flight crew training and re-certification when previously had not existed. So its TCO cost has risen.

It's been pointed out they've cancelled Airbus purchases too. This also strongly suggests what I thought about PR value is irrelevant: its not about marketing, its about real money. They may well want the Max. they can't afford it.