The reason why Stansted has suffered so much is because BAA havn't put in the amount of effort it deserves. They have sat there on their backsides and done nothing but put up a fight for an airport whilst they watch it slip into decline and done nothing to try and go about it except consistantly appealing the courts. Clearly just a bit on the side purely to help Heathrow out finincially.
Once MAG take control, Stansted will be in very good hands. Stansted will probably be the London airport with the strongest owners in a sense as MAG have worked with all the different types of airlines, from LCC's, to holiday charters, to full service flag carriers in large amounts for a very long time, giving them great experince and understanding in what each type requires to find their airport attractive.
I'm sure MAG will bring those valuable skills to Stansted and transform it back into it's former glory and back to the position where it should be. GIP and Gatwick have done it but the difference is, they can no longer take advantage of Stansted's poor and uncertain ownership.