I disagree completely. DS is completely pointless task on the east coast. If you want to double stack west of parkes then fine but there is no reason at all to double stack north, south or east of parkes or on the main south. Dual stacking is never going to happen in to Sydney. The inland makes it even less required since you reduce your travel time from Brisbane and freight to Melbourne is still going to go through Adelaide and up the hills.
Duplicate the inland instead and allow more capacity in this regard instead. DS west of parkes if you wish since from there the traffic can be divided up or combined to/from Brisbane, Sydney, Newcastle, Wollongong or elsewhere. This is again another red mark against inland rail and the ARTC where again they aren't investing in the correct solution.
One more time
DS from Perth to Parkes is not a destination and adds extra complexity to the train loading at Perth, the destination is Melbourne, Sydney and Brisbane. Whether DS between Melbourne and Brisbane is viable is almost but not entirely irrelevant, however it does help and why have just a few bridges as your only impost to providing this option?
DS to Western Sydney is not far from reality, post Inland there is only a few bridges and structure to be upgraded, some maybe be done as part of life cycle replacement others for DS project purposes.
DS is not about increasing capacity as so much about reducing the cost of running an individual train, this is why they double stack to Darwin despite the lack of traffic on the line. Fixing a few bridges is nothing compared to building hundreds of km of track.