Considering we're talking about Boeing, who actually do the work Spirit does for them for the 737 for other airframes (making the fuselages), and used to do it in house for the 737 before too (in that same Wichita factory complex where Spirit is now, Spirit being the result of Boeing's monumentally stupid decision to divest), I think it's fair to say that Boeing can bring production back in house.
These sorts of what I like to call "reverse monopolies" are all over the place in US industry now. So many companies dumped out pieces of themselves to a single company that then supplies for all of them, removing the ability of those companies to actually compete with each other by differentiation.