The old SEx/MEx sleeper service was better than the XPT ever was. You had the full deluxe cabins if you wanted it with families, a proper dining car, and the slower travel time actually made more sense for overnight.
The problem with the XPT is that it's always been a weird middle ground. It's not a high speed train or anything close, it's just a bit faster, and the road these days is in a state where it's a reasonably ok one day drive if you've reason not to fly.
In full at: https://eresources.hcourt.gov.au/showCase/2023/HCA/37