Airbus and Boeing are widely considered to be the one of most competitive and efficient duopolies on the planet. That might have a lot to do with the fact that

Airbus and Boeing are widely considered to be the one of most competitive and efficient duopolies on the planet. That might have a lot to do with the fact that they are based in two different large economies and cultures. The nature of their products and customers, and the scale needed, may not lend itself to having a lot of smaller competitors. Where are the economies of scale (or other efficiencies) going to come from? This may already be reasonably optimal for large aircraft.