You've got that backwards. There is no downside. Software does not an airplane make, even though there are now attempts to use software to fix airplane design issues.
You wrote 'Airbus' as though Boeings R&D in their software would somehow magically translate into an advantage for Airbus. But Airbus should also open source their code, and for exactly the same reason. In fact Airplane certification institutions such as the FAA and counterparts could easily mandate the open sourcing of every last bit of software to create a level playing field.
But Airbus has been doing the computer aided flight longer than Boeing so mabe it would be more push back from airbus to not give Boeing any free code?
The whole idea that you could not open source it with such restrictions that a competitor could not use the code is so strange. It is perfectly possible to open source code in such a way that you can't use it for free in your own commercial product without a license.
The whole idea that by open sourcing you will automatically turn their code into code that maximizes social prosperity (safer? cheaper planes? doubtful) is so strange.
You wrote 'Airbus' as though Boeings R&D in their software would somehow magically translate into an advantage for Airbus. But Airbus should also open source their code, and for exactly the same reason. In fact Airplane certification institutions such as the FAA and counterparts could easily mandate the open sourcing of every last bit of software to create a level playing field.