It's not simply the change of ownership, although a new management will make a difference to the appeal of the airport to some airlines depending on what changes they make. The key thing though is that Stansted is now seperately owned from Heathrow and it's ownership is no longer in doubt and is stable for the forseeable future. That itself will encourage more long term commitment from more airlines. So actually, the change of ownership does make a huge difference in a variety of ways.
Aegean Airlines
Air Arabia Maroc
Atlantic Airways
Blue Air
Cyprus Airways
El Al
Norwegian
Turkish Airlines
WOW Air
These are examples of carriers that no longer serve Stansted but still serve London. None of these carriers moved airports because they couldn't get enough bums on seats at Stansted, they left because they could see better potential financially elsewhere. Others have been and gone, some of which moved around a bit before going, but that doesn't mean they won't come back. Stansted's new owners spent £1.5 billion because they clearly believe they can bring more traffic back. It's not simply the case that the market decides the future of Stansted, that could apply to any airport. It's the people within the market that decide and MAG is a big part of the market.
In today's economic climate, many airlines are not in a position to take excessive risks just to see if a route is viable. Indeed, as your favorite orange airline have proved, carriers will simply move aircraft around to achieve the best given return for that asset.
Contradicting yourself there a bit aren't you?