Boeing certainly bears culpability for failing to execute, and whatever terrible lapses in certification were made to get there.
But why were they doing a dumb thing in the first place?
Because American Airlines asked-told them to. (Because they didn't want to have to pay to retrain pilots)
Boeing deserves to have substantial portions of management jailed over this, but there's blood on the legacy 737 operators' hands as well, for asking them to do it in the first place.
This sounds like the ratings agencies during the last recession, where they knew they were handing out good rating for junk bonds because if they didn't do it they'd lose a customer to the competition.
It's totally fucked.
Boeing was caught flat-footed and underinvested in a new product in 2011.
So while Boeing management could have said "No" to their largest (?) customer, that would have been a hard decision to make. And probably would have led to the board chopping C* heads for breach of fiduciary duty.
The Airbus order backlog is almost a decade out as it is, so American just shoving it all into A320neos that would take longer to show up wouldn't exactly be realistic either.
Boeing should've just gone with a clean-sheet design.
Hindsight is 20/20. The CEO chose a risky payoff (Boeing remaining competitive with the Max) over certain loss (Boeing losing many contracts to competition).
If the CEO didn't expect jail time (which given our history of such incidents is unlikely), he chose rationally in his own self interest.
If you want a different outcome you need to disincentivize this kind of behavior. E.g. if CEOs "taking responsibility" (because that's usually the reasoning for their insane income) meant they get locked up in prison without parole for 20 years. Then rational actors may become more careful.
Of course every customer ever is always going to ask for products on a faster timeline at a cheaper price. That doesn't absolve the manufacturer of its responsibility to build a safe product.
And I'd be inclined to agree it's 100% manufacturer fault... except this was a pretty high pressure ask.
IMHO, for me, it crosses over the line into sharing some of the blame.
Yeah, because American doesn't deserve blame for this nearly to the extent that Boeing does.
In December 2010, Airbus launches the A320neo, featuring new engines (LEAP included).
In July 2011, American Airlines releases a press release  containing the following:
"As part of the Boeing agreement, American will take delivery of 100 aircraft from Boeing’s current 737NG family starting in 2013, including three 737-800 options that had been exercised as of July 1, 2011. American also intends to order 100 of Boeing’s expected new evolution of the 737NG, with a new engine that would offer even more significant fuel-efficiency gains over today’s models. American is pleased to be the first airline to commit to Boeing’s new 737 family offering, which is expected to provide a new level of economic efficiency and operational performance, pending final confirmation of the program by Boeing. This airplane would be powered by CFM International’s LEAP-X engine." (emphasis added)
In August 2011, Boeing announces the 737 MAX program, featuring LEAP engines.
American Airlines literally ordered a plane that didn't exist. And then Boeing tried to build it.
There are structural failings (e.g. why Boeing wasn't better positioned by investing in a redesign in the 2006+ period), but American shares a fair amount of blame for this clusterfuck.
Boeing certainly could have said "No", in which case American likely would have bought additional planes from Airbus. So American had leverage, they used it to pressure Boeing into building what they wanted, Boeing failed at delivering that, and we're here today.
Previous to American's press release, Boeing's leadership was strictly avoiding mentioning an up-engined 737. And was fairly reliably mentioning a clean sheet redesign program.
I can't think of a reason for doing that, if they in fact had such a program.
It would make sense to be trumpeting it loudly to anyone and everyone.