the problem for Boeing was that Bombardier was launching the C-Series. Airbus recognized both a threat and an opportunity and launched the Neo at very low R&D cost. AA among others wanted that.

Boeing faced a difficult conundrum. Do they develop a clean-sheet design that would be more efficient and would leapfrog the Neo but take 4-5 years longer to design, thereby leaving the NG (Boeing’s bread and butter) severely exposed to an aircraft with far better economics or do they simply follow Airbus with an expectation of retaining same market share split.

Boeing has a very conservative corporate culture that focuses on returning cash to shareholders. The low-risk choice was to follow what Airbus was doing. A clean sheet design not only left them vulnerable for too long but would likely use the same generation of engines as the Neo, so would only be marginally more efficient unless Boeing were to go to composites, which it has never done at that kind of volume. A new composite plane would be more efficient but also far more expensive to build and amortize the development costs. I think Boeing was having a hard time making the numbers work.

What they missed (besides the MCAS problems) was the move by the market to larger narrow bodies, where the A321 has a significant advantage over the original MAX-9 and the -10.