Car platforms change a lot more, a lot more often, IMHO.
I don't know if such a state of affairs is possible with the aircraft or airline industries. Those industries are too rational, whereas the expense of safety features in the consumer automobile market is in part quietly covered by arguably irrational consumer spending habits.
Break them up and it's easier for both the market and the government to punish bad behavior.
They all have the rights to the existing designs, and can create derivatives, but they don't automatically get the rights to the others' derivatives, so they have the incentive to specialize or develop new designs to find a niche, for the same reason the Boeing 737 isn't an exact clone of the Airbus A320 or vice versa.
If there were more competitors, they'd lobby to change how the regulations work so they didn't effectively destroy the market for new aircraft types.
It was a beautiful aircraft, destroyed by years of management comittees.
Boeing needs to be broken up and reorganized, full stop. Congress has to act. It is simply too important and vital of a company to US strategic interests to be left in this state.
SV has tried and failed time and again to realize the enormity of undertaking that is commercial aviation. There are less than 10 countries in the world capable of producing and marketing a competitive modern civil airliner. And of those, a vast majority are state nationalized conglomerates, or in the case of Airbus multi-country efforts. It takes almost the entirety of a country's aviation workforce and industrial capacity to maintain just one of these companies. Boeing cannot be replaced. It has to be fixed.
That's more like Microsoft's kind of market, but uh... don't give them any ideas.
The MD merger that led to the shift of Boeing management culture was due to the government intervening and forcing a marriage between the two companies because MD was too important for vital defense interests.