I would say the reason it happened is that Boeing put Harry Stonecipher in charge, with many other McD D executives also put in high positions of power. Enough that commentators noted that it was unusual how many of the acquired company's executives wound up running the show, relative to the acquiring company's executives.
IMO, your argument is that the average-ization of Boeing was inevitable due to the trends and forces theory of history. I don't think it was inevitable, but the result of conscious choices that were made initially at Boeing around the time of and in the years after the acquisition, and subsequently by originally-McD D people who were then in positions of power at Boeing.
I'll grant that without background on Boeing and McD D, it probably looks like it's just another company following the corporate trend. But Boeing had been an exceptional example of consistent quality engineering for decades, including decades where major automakers in the U.S. were doing the typical "just enough in engineering to satisfy the marketing needs, only what is required by the government in safety and lobby them to prevent any more of that, pay out maximum profits to shareholders." Example A of such American automotive negligence while Boeing was excellent:
The Oldsmobile diesel. Other exhibits include all the rusting out (convenient planned obsolescence!) even after solutions to the problem were available, the Ford Pinto, and the extended resistance to seat belts.
I also think competition of lack thereof plays a part. Why did the Big Three automakers eventually get less bad about quality/reliability/safety*? Largely, there lunch was being eaten by Japanese and later Korean, Swedish, and German imports. Today it's not too hard to avoid a 737-MAX, but part of the reason airlines keep buying them is the lack of alternatives that can be delivered in time to replace their aging existing fleets.
* - Still wouldn't buy a Stellantis (aka Chrysler, aka Fiat Chrysler) product though