Hmmm... Most of this thread doesn't even talk to the topic description.
The primary advantage of a 'metro' is in the non-use of pantographs for power pickup, using a variant of 3rd rail instead, and significantly reducing the size/cost of tunnels to be bored - if its loading gauge can also be smaller, so much the better. If the NW loading gauge is smaller (?) , using this rolling stock on a conventional standard gauge line would result in even wider gaps at stations.
As to the 'two stations for 1.8 billion' tag, that amount also includes a 12 track/20 train stabling yard and a very significant upgrage to Leppington and associated 330kv/1500v electrical system, so the two stations are a lot less than 1.8 billion. The quality of the overall line is very good to my mind, and if the two stations had not been aimed at some 'architectral prize' but limited to the average functionality one sees in the rest of the Sydney network, the stations themselves would have been a deal cheaper I suspect.
O/H pickup doesn't impact on tunnel size apart from old technology tunnels. These days we don't tend to build London tube type tunnels where you feel like you are in a pipe not a train. Have a look at some systems, fixed beam instead of a wire and the pano only raises slightly above the cabin.
NWRL was to use O/H because it existed on the ECRL and the costs of conversion to other was not deemed worth while. When you look at the infrastructure required for side rail pick up (no one builds a Greenfield track with a live exposed 3rd rail anymore), I think O/H must be similar costs. Being 1500VDC over 750VDC there would be some subs station advantages as well.
The last single station extension of the Gold Coast Line cost as much as the initial project, but thats because they had to move an old tip and other 2ndry actions for the area that opened up for more development. So yes $1.8B is more than just two more stations.
I believe the plan is for DD technology to approach the airport from the east and the NWRL to be extended down with connection from Western line and then onto airport. Sort of similar to Munich which has a loop to the airport and two different lines arrive at the airport station, one each platform.
For the doubters, the airport will be built. Sydney Airport is heavily constrained by the curfew for international flights. Forcing large scale arrivals in a small window before peak. Also forcing cargo to arrive during the day. Thus making international aircraft scheduling more complex for the airlines and limiting the speed of air freight. International flights need to arrive at Sydney about 90min before the start of the evening curfew so they can get turned around and take off and avoid the cost of having a plane and crew land locked in Sydney for 8hr instead of making money flying.