I think that the MAX was about as modest as they could make it. That was really the point of the whole exercise. A more modern engine core with a smaller fan wouldn't have moved the fuel economy needle enough to justify a new model I don't think, and Boeing would have lost a lot of orders to the A320neo.
Everything would have been fine except there was so much pressure to make every change as modest as possible that it didn't quite all work. They needed to either make significant changes to the landing gear or significant changes to the flight control computer and they chose neither.
Everything would have been fine except there was so much pressure to make every change as modest as possible that it didn't quite all work. They needed to either make significant changes to the landing gear or significant changes to the flight control computer and they chose neither.