I don't think Capcom would profit at all with a Microsot buyout. Franchises like Monster Hunter and Phoenix Wright need to be on handhelds, and their big series (Resident Evil, Street Fighter...), work well on any system, not just Xbox. So a buyout would heavily limit their audience, specially if they try to force the japanese market to buy an XBox (which is, at this point, impossible).
And besides, wasn't there a law in Japan that made buying japanese companies extremely hard for foreign companies?