If recent history is an indication of MS intent they don't want to own any studios. Why chance potential losses in a very risky business? Exclusivity has been handled with Bungie, Bioware, Epic etc. without outright ownership so far. Why would they potentially create a less efficient company that takes more risks with MS money? Innovation will suffer long term.
How many of Sony's 1st party devs are making profit and how much? The hd twins operate in a different world from Nintendo where user base and dev. costs are big positives. A look at the list of failed devs. in 2009 shows that making good games guarantees nothing.