I don't think there is much to worry about Wizz Air moving, but the management at the airport do need to keep in mind that there is a possibility of another airport could come along and offer them a better deal with better facilities!
I also do not see the threat by Ryanair as smoke and mirrors, and they have withdrawn from many airports that didn't meet their demands and prices they wanted to pay! Dublin, Shannon, Madrid and Alicante all come to mind...
While the airport is pretty full, they have had the opportunities to attract traffic from STN from the likes of German Wings, Norwegian and Air Berlin, who do not base aircraft at any London Airport...
Also Thomson and Monarch have removed an aircraft or two from Luton in recent years, so there has been space to add a few Ryanair aircraft, but they haven't.
While airlines usually agree confidential terms and prices to fly from an airport... Would anyone know Luton's published fees and hows these compare to Stansted and Gatwick, and if they are really that attractive?