WOTC - Build terminal 5 when airspace is limited

I asked this question in the Wisdom of the Crowd:

Why are we building terminal 5 when our airspace is limited?

Here are the responses: (49 Votes)

84 % - We are quite wasteful in spending on infrastructure.
10 % - Terminal 5 will allow greater efficiency for the airport.
4 % - The planners probably overlook this important consideration.
2 % - The limited airspace is due to weather condition. We can have more planes on normal days.

See the pie chart at: 
http://www.wisdomofthecrowd.sg/chart.aspx?ID=1020