You say that as if that's an excuse. What Boeing's customers wanted is irrelevant, but in any case, your claim is incorrect.
Airlines just want a plane that can make them money. Boeing could have build them a plane much better than the MAX, but they chose not to because if they had done so, then the airlines would have the option to switch to Airbus. The Airbus option would not be on the table if Boeing was just selling them another 737 variant.
What about the impact of a new model on the ground for several months, maybe a year, even maybe (with a tiny chance, of course) forever?
I bet it would have been cheaper if they started adding a new model to their operations already (for those who don't have it already)
Sure, customers always want the cheapest option, and it's up to Boeing to say when enough is enough.
In hindsight, yes, but Boeing had been modifying it for fifty years, so I get why they thought they could do it again.