I think Kl911 raises a valid point here. The construction of the wing is a key piece of technical know-how in building airliners. Proprietary knowledge and processes are a source of competitive advantage. That said I am not all that worried that Mitsubushi is going to run Boeing out of the market.
Japanese firms have been careful with exactly what production processes they send overseas. Sharp has went further than that for example by introducing certain manual steps into their flat screen TV
production process that could be fully automated. That way they can ensure (or try to) that a rival simply does not buy the same machine tools and compete against them.
I think the story linked above is very relevant because it raises the question of what Boeing will be in future. It looks they are moving away from manufacturing and more into the role of a leading prime contractor and marketer. Given the important role of aerospace in national defense that does potentially raise some interesting issues for the long term.