MAG are not going to throw money at STN for the likes of a 2nd runway (as the Govt and Boris hope) unless it brings in lots of dosh from big full fare airlines .
Nigel,
You're absolutely right, putting national infrastructure, such as airports, and the railways into private hands ensures that the government has limited strategic control over it's development. However that is an ideological decision that was put in train by a certain "late" prime minister and her government - and subsequent governments have done nothing to change that model.
That said, MAG is hardly a private company, more a quasi independent arm of local government in the Northwest. One of the few "nationalised companies" left in UK.