Probably the biggest barrier to a new creating a new commercial airline manufacturer is that there just aren't that many new planes sold each year. There aren't

Probably the biggest barrier to a new creating a new commercial airline manufacturer is that there just aren't that many new planes sold each year. There aren't that many customers for commercial airplanes, and existing airplanes can last for decades when properly maintained. Combine all that with the inherently high costs of running a commercial airline manufacturer, and there just isn't enough demand to support more companies in the space. Changing that would require huge technical breakthroughs, or fundamental changes to how passenger air travel works. Neither of those seem to be likely in the near future.