Lots of interesting hypothesis here ...............
I rode on the Indian Pacific a few years ago and had the chance to discuss GSR's train offerings with the train manager on that journey.
Yes they do remember the Southern Spirit. It was hoped to be a continual service but it ended up not working out that way. No matter as they were running short of coaches for the IP and Ghan so they deleted the Southern Spirit.
I asked if it had a future. The answer was the company would do reviews of all services and should demand show a Brisbane - Adelaide train was needed it may be restarted , , according to need.
Given the number of repeat passengers on the IP that I met, it could mean the Southern Spirit runs again for people who are willing to spend on a train journey but don't want to go for a third or fourth journey on the IP or Ghan. They may want to see something different.
there's always another "but"
I rang GSR on the day this thread was posted and asked them if there was any Brisbane - Adelaide train services available or being contemplated any time is the next couple of years.
As far as their booking call center was aware the will be no Brisbane to Adelaide any time soon. So my positive side hopes the call center operator has not been advised of a new service , I will book for 2 if it becomes available.
My negative side says we were all lead on a wild goose chase.
Wait and see .............