LTNman
I would suggest that what went wrong was "events dear boy, events" as Harold Macmillan memorably said many decades ago A near doubling of predicted passenger numbers over ten years during a period, which 2005 certainly was within, of considerable growth in air travel doesn't strike me as being a "fantasy". I'm sure there were many other such predictions/assumptions around that time which were much more optimistic than Southampton's.
I've never placed much faith in growth predictions as they are subject to matters totally beyond the control of airport operators. Numerous time I have been asked to produce a three-year growth prediction on a new air route but have always balked when asked to extend it further to five years as I saw that as being meaningless.