The "funny" thing to me is that I'm old enough to remember when the merger occurred that people were predicting exactly what came to pass. It just took 25 years to finally happen.
"Quality inertia" is one thing which allows for giant amounts of damage to be done long before the wheels visibly come off, as deviance gets normalised (and even mandated in cases such as Boeing) and the company eats its reserve of quality and goodwill, it starts going off-track in small ways before it falls off a cliff.
It took closer to 20 years than 25 for the wheels to come off of Boeing. Lion Air 610 crashed on October 29, 2018, the MDD merger was on August 1, 1997.
And that crash was the first major externally visible symptom[0], the decisions which led to it happened years earlier (2014-2015).
[0] unless you count the dreamliner's joke of a rollout, which probably should have been the warning shot that things were getting unwell