Prestwick is really showing it's age now, a major rebuild will be necessary soon but it's a bit catch 22. With modern, well designed facilities PIK could attract more airlines and therefore make more money and invest in improving facilities. But a rebuild costs money, and to make more money more airlines and passengers are needed, but with the current facilities, doing that will be hard and so the cycle goes on.
It will take a brave person in this financial climate to commit what could end up being a couple of hundred million pounds into a project that may not have a return for a considerable length of time.
On an unrelated note, what do people think the market from Prestwick (or Glasgow) would be for city routes (Paris, Amsterdam, Berlin, Scandanavia etc) using 50-99 seat aircraft would be. Ryanair used to with 189 seat 738's so a smaller operator using something like an E-Jet or an AN-148(!) May have some joy if marketed correctly.