We already have several other players that have experience building aircraft: Bombardier and Mitsubishi come to mind, and China has a manufacturer that's keen on challenging the duopoly with their jet too.

Don't forget, building a jet isn't that big a deal. Boeing doesn't even make the 737MAX fuselage! It's built by a company called Spirit in Kansas. Boeing doesn't make the engines either, of course (no aircraft manufacturer builds engines, which are surely the most complex and technically challenging part of the plane). Boeing does the engineering, the wings I suppose, and the final assembly.

It seems like it shouldn't be that hard for a smaller competitor to get a capital infusion and scale up to building larger jets.