Even if the line isn't planned to operate overnight initially, it is unlikely the significant number of international arrivals & departures during the night will change due to the timing of connecting flights to/from Europe. So to allow for a future extension of operating hours as air traffic rises, will there be bidirectional signalling between Forrestfield and Perth? This will allow a reduced overnight service to operate on one track while track maintenance is carried out on the other.
The key to running successful public transport without burning taxpayers money is knowing when to run and when not to run.
The number of OS flights to/from Perth outside the published operating hours for the new line is likely to be maybe 1 or 2 per hour. How many people on these planes would use public transport at 2-3am in the morning in a city they are not familiar with? Most are street smart enough to know not to do this. I went to Oslo 2 years back, arrived off airport train in city at 2am, caught taxi for about 750m because 1, not familiar with streets and didn't have map which way and 2, safety. Its a safe city, but I don't know if I am walking down a bad street or not.
Most would have the common sense not to. Additionally the bulk of the rest of the train/bus network will probably be closed so the transport options will be limited. Additionally how many passengers is 1-2 planes per hour likely to offer and would you run even a 20min service for them? The right decision to close the airport line around same time as rest of network? These pax will bus/taxi/private transport/hotel transport it.
A few international airport rail services that I have used
Brisbane - closes for roughly 8hr a night
Dubai - 2nd busiest airport in world. Metro closes between midnight and 6am, starts 1pm Friday (first day of 2 day weekend).
Hong Kong - airport rail service Opens around 6am
Singapore - I believe is also capped in operating hours
Oslo - stops not long after midnight
Munich - does not run all night
Vienna - I believe closes during the night
Geneva - Closes during the night
Madrid - When I was there 12yrs back, Metro and assumed Airport closed just after Midnight
Vancouver - As above
Copenhagen - I caught at taxi at 4am Friday morning, because the hotel told me there was no train and I was 500m from main station. Note, I walked out the door and was immediately met by 10+ prostitutes with plenty of others further away and you could see their pimps standing further away. Yet it was a nice hotel on a nice street and at 10pm was still lots of families around. That 500m walk would not have been comfortable.