Real truth is that it is a cost saving gimmick.
It's not a cost savings for Boeing - they wanted to put the 757 flight deck on the 737ng for manufacturing commonalty and cost savings 20 years ago. The cost savings is for the operators for crew training.
Boeing really wanted to replace the 737ng with an entirely new airplane instead of doing the Max, but Airbus forced their hand with the NEO. Not only would an all new aircraft required a much longer development time, it would have taken several years more after it was certified to ramp production up to the current 40 or 50 aircraft/month. That would have basically meant conceding a 500-600 aircraft/year market to AB for many years.
My personal crystal ball says Boeing will launch a new airplane program in the early to mid 2020s for the 150-250 passenger market - basically a 737/757 replacement. I wouldn't be surprised if it's
not single aisle.