Regardless of the Government's forecasts, all of the affected airports are operated by commercial companies, with the usual commercial constraints. The normal tendency is to try and build major infrastructure (terminals, aprons or runways) just in time. This is why a terminal extension or replacement project is usually preceded by a period of uncomfortable overcrowding - you must have noticed! Airport companies do not want to undertake mjor construction, just to have it lie unused whilst costing huge amounts in interest and maintenance/running costs.
Given the long lead time in planning for such projects, airports have plenty of time to reconsider, and often delay, modify or even abandon plans rather than waste money. The "huge waste" scenarios mentioned above are only likely in the event of a sudden collapse of the market, occurring just after the investment is complete.