I am no fan of WAC but...
The original expansion plan for Perth airport went out the window when WAC took over. To make a parallel runway where it was orginally intended you would have to bulldoze the Coles and Woolworth's distribution centres (and others).
is not quite correct. A large area of land has been left for a "short" runway parallel to 21/03. At present it is sealed with cars on it but no buildings as yet.
The difference is that when aircraft leave Gatwick they're heading to the four points of the compass. When aircraft leave Perth they're generally heading in one. I suspect that it's not runway capacity that is the issue but airspace capacity.
This is simply not true. Aircraft leave Perth to track out through an arc of about 200 degrees. I think Gatwick would also have about four main departure tracks.
Part of the issue is that 24 is not used for departures in a morning due to noise abatement. So a perfectly useable runway is left mostly idle during the morning departures because a couple of suburbs (South Perth, Como?) might hear an aeroplane.
The fix is turn Pearce into a multi user facility the same as Darwin and Townsville and extend the Midland rail line through Ellenbrook to Pearce. It will never happen.
How far along is Terminal WA now?