Boeing was simply trying to milk the 737 for all it was worth, squeezing the last drop of profit from 50 years of grandfathering. Its only competitive advantage over the A32x family was the faux CCQ - Cross Cockpit Qualification. Airline managers faced with a new type certification for their crews for EICAS would then immediately consider the A32x family as coming in to play. If you are going have to type your crews anyway, your degree of freedom just increased.
Boeing was actually able to monetize this lack of training, capturing some of this value for itself by charging a higher price for an old airframe whose development costs had long ago amortised to zero. It extracted an economic rent - a profit beyond fair value. If crews had to be typed on, an airline manager would simply assign a lower future value to the new aircraft to take into account the additional training costs, and Boeing could not have extracted that rent.
Having flown in a mixed classic / NG fleet system with a 90 day recency requirement for each model. Quite frankly it was a nightmare for all involved, it imposed a large inefficiency onto the organisation and crew. I can actually understand why airlines such as Southwest would write a $1 million penalty into a MAX contract if additional simulator training was required. They would not want a mixed fleet from a crewing perspective.
Having said all that, Boeing pushed the system too far. The $10 billion in industry costs for an EICAS model is now a line ball breakeven, and 346 people have lost their lives. Even worse, the 737 line is now done, the MAX will be the last 737 model. Airline managers would look to any new narrow body from Boeing as a risky proposition compared to the mature A32x family given all the issues with the entry into service of the B787. In effect, Boeing has devaluled the future value of it's future narrow body model. It will have to offer a steep discount the A32x family initially to gain the confidence of airline managers.