1. Aircraft is not a monopoly, it's a duopoly between Boeing and Airbus in certain aircraft segments, and there are other manufacturers like Embraer. Airbus has manufacturing lines in the US and sells plenty of aircraft to American customers so it's not even a monopoly on manufacturing in the US.

2. Aircraft development has always been really expensive. Lots of iconic companies have shuttered, merged, or otherwise ceased to exist because aircraft development is fucking hard. Historically:

* McDonnell Douglas merged into Boeing (or the other way around, depending on your view) after they failed to build the MD-11 up to the performance advertised to customers and suffered billions in costs and delays, and that was formed from an earlier merger of McDonnell and Douglas after Douglas struggled with the costs of developing the DC-8 and 9, both airliners that are considered to have sold fairly well.

* Airbus was formed from scavenging various European national aircraft manufacturers because they would all lose money if they tried competing individually vs MCD and Boeing.

* Lockheed Martin exited the commercial aircraft market decades ago after spending billions of dollars on a delayed aircraft that airlines said they were interested in, but then got cold feet about buying.

More recently:

* Bombardier sold its aircraft lines to Airbus because it spent billions of dollars trying to fix mistakes and delays.

* Embraer tried to sell stakes in its aircraft lines to Boeing because despite having airplanes that sell well they could not cope with the cost of developing updated versions of their aircraft.

* China, Japan, and Russia have each had state-backed programs to try and take some of the lucrative airline market. All of the programs have cost billions of dollars, were delayed, and on top of that non-domestic customers are not really interested in these planes since they have worse efficiency and performance characteristics than competing aircraft products.

Even the big two have had problems; every single recent aircraft line has seen delays and cost overruns. Boeing and Airbus are just big enough that they can eat the cost of it without needing lots of government subsidy, but the MAX8 crisis has shown that might not even be true anymore.

If it were possible to reduce costs, someone would have done it by now, because aircraft manufacturing can be a very lucrative industry resulting in millions of direct and indirect jobs. As it turns out, aircraft are expensive because the ever-increasing requirements for fuel efficiency and safety from airlines and regulators have resulted in aircraft that get harder and harder to develop and build each generation. Rolling any of this back is not a good idea, because

* for various reasons, using more fuel than you have to for air travel is not good

* every single safety regulation for aircraft is written in blood, and in general people do not want to fly in planes created via "move fast, break things"; see the MAX8 crisis.

> Monopolies also aren't allowed to do certain things no matter how they got that way. (Many of the things Apple does come to mind.)

What is something Boeing has done that they should not be allowed to do as a result of their position in a duopoly?