Interesting thread with many different opinions and some good discussion.
To throw in my two cents worth...
I've taken The Overland 6 times in the past three years, twice ADL-MEL and four times MEL-ADL on a mixture of medical and business trips, and have booked MEL-ADL again for a return from a business trip this coming February. Was a big user of it back in the 1980's during my University years, when for the relatively measly sum of around $50 my mates and I could do a day in Melbourne with overnight rolling accommodation.
In recent years it's always been my preferred mode of travel provided I can spare a day at the beginning or end of my time away for work or medical reasons.
Of these recent travels, the train stopped at Bordertown five times to set-down passengers. Not many passengers, granted, typically just a sole passenger. Certainly not a hub of activity there - but important to the people who did disembark.
And of the two journeys I have taken ADL-MEL, on both occasions multiple people boarded at Murray Bridge, destined for MEL. Surprised me a bit. So there may be some need there.
With regards tourists using the service as a connection, I have first-hand experience of just that, when in Feb this year I got chatting with a retired couple from the UK who were going MEL-ADL to connect two days later with the Indian Pacific. Again, just a single example, but this couple did spend a lot of money in wineries and restaurants in the Southern Vales while they were here - I know because drove them around in my "classic car" on the Sunday. I'm a very gregarious traveller and it's amazing how easy it is to make friends on The Overland when you offer access to your personal wireless access point to passengers who discover only after boarding that The Overland doesn't have Wi-Fi!
I'm a realist and understand that The Overland is running beyond its means without government support. These aren't the heady days of the 1980's when nightly service still saw three or four cars of each class on each service for economy, first sitting and sleeping in each direction. Or the two months in late 1989 when AN ran even larger Overlands full to the brim because of the pilot's strike.
The question in my mind is whether it is a public transport service or not. Public transport requires government funding to operate as it is a service to the public. Rail cruises - to use GSR's terminology - don't qualify. And The Overland these days is just a rail cruise.
Maybe one day it will be replaced by a more public-transport oriented service. It won't be as comfortable as the current incarnation, but it may be a little faster and operate regularly enough so that one needn't plan one's travels around it. That will require government subsidy I'm sure, but I'd be all for that.