it's worth pointing out, however, that it was a software-only feature that they turned on or off depending on whether the customer had paid. If I understand right?

I think that changes the calculus of "Do you pay 10x the price of an aircraft to reduce the chances a problem so that it goes from happening once every 100 years to once every 1000 years"

They had already invested the money to develop the feature. Delivering a plane with or without it to a customer had the same cost. If the customer paid more, they got the safety feature turned on, the end.