> If Boeing wanted to be responsive to those customers they
  > could have produced a plane that didn’t need additional training.

The 737 platform could not accommodate the more efficient, larger engines in their original location. In order to add these engines, they had to be placed in a different location and thus the plane handles differently. Thus, the requirement for additional training.

There was no way to create such an efficient aircraft on the 737 airframe without requiring pilot training, even if software could smooth over the differences 99% of the time.