"If airports were such a poor financial prospect you wouldn't have significant numbers of them falling into the hands of various financial organisations very much at the speculation end of the scale."
Buying a failing airport doesn't cost very much - and when it becomes uneconomic you get a vast area of open, flat LAND that you don't have to decontaminate, drain, is normally connected with reasonable roads or even rail links and with locals who have no great love for the noise etc from current operations. A developers dream TBH