Its also worth remembering that a runway extension actually benefits the current users more than any possible future ones. Due to the hill on the 20 departure many aircraft types, including the ATR / EMB (as stated by LM) are pretty badly restricted, especially in hotter months. Whilst all airlines operating into SOU obviously make it work at the moment, its not to say everyone wouldn't benefit from an extension, plus now is a great time for investment as those working on the runway extension no doubt get paid a salary, pay taxes and of course spend that hard earned money (possibly?) in the local area boosting the Hampshire economy.