Consider how long it took them to complete what was actually built. We'd still be waiting if they went with a top/bottom setup, lol.
Gonna have to disagree with you there. Having the lights go red when there are no pedestrians is worse for congestion than having a zebra crossing. The problem at T3/4 is you have two lanes of traffic merging into one, that's the major cause of congestion when it's busy. When it's busy at T1, you end up having two lanes of traffic trying to get across to the first 12 bays on the left. Having bays on both sides of the road would have been a much better idea.
In my view, T2 is probably the best setup at Perth Airport. It's very compact, it has more bays that any of the other terminals, and it's extremely easy to circle around again if need be because of the roundabout at the entry/exit point.
Perth is ranked 75 in 2016, compared with Brisbane at 17, Sydney at 23, Melbourne at 25.