The key 737 Max decisions were made circa 2011-2013, while James McNerney was the CEO (2005-2015). That was when the design objective of no simulator training required was set for the plane (and Boeing released marketing information promising that) while also promising to do it with engines that were too big for the wings. That decision was what led to everything else, and was loudly announced and marketed while Muilenburg was over on the Integrated Defense side of the company.

James McNerney was a Harvard MBA who spent time at McKinsey before going to the Jack Welch clown show at GE, where he ran the aircraft engine business for a long time. When he lost out to Immelt to be CEO there, he went to 3M for a few years then bounced to Boeing.

Now, Muilenburg has to bear the mistakes he made: pushing an unsafe plane through certification, not listening to the right people, not investigating the Lion Air crash sufficiently, etc. But the evidence I think strongly shows that someone who was not an engineer was making the final decisions on the "bet the company for the next 10 years" choice to go forward with the 737Max rather than a clean-sheet design, back in 2011.