Have you seen the size of Terminal 5?? Its the same size as the current central area...... and they've been working on that for 75 years and it's still cack!!
There is a lot more to Terminal 5 than the big shiny building in the middle....... did U know it goes into the ground almost as much as it goes into the air?? Imagine how much infrastructure goes into something that size!!
Hence......a long time to build!!