Two of which could be quite easily improved upon.
of which (service frequency) is fixed easily but not cheaply.
The speed problem is not easily fixed, with a substantial amount of work needed to get the rail network up to the standard needed for the Overland to be faster than a coach, let alone at a point where it's faster than a car and starting to compete with a plane. Remember that with a car there is no transfer at either end, and that real people undertaking real journeys don't generally make the arbitrary city centre to city centre journeys used by pro-rail people to tilt a comparison in their favour.
Population catchment is probably not that different to Melbourne - Sydney ...
Adelaide: 1.3 million people
Sydney: 4.9 million people
And that doesn't account for the significant proportion of Sydney-Melbourne XPT passengers who are using it as an inter-regional service to one of the centres along the way rather than an inter-city service. The only significant regional centres on the Overland route which would come close to warranting a stop if they were on that XPT route are Ararat, Horsham and Murray Bridge - and two of them have other superior public transport options linking them to their respective state capitals.
For most people, the thought of needlessly spending an entire day on a train, when there is a 1 hour flight as an affordable alternative, its pure insanity. That is the only reason the Overland has declined, and is the reason why the Overland will continue to decline.
There is plenty of research in Europe to suggest that high speed rail can compete against air only when the train journey time is a maximum of two hours longer than the flight time - largely to account for the time lost while the plane is on the ground at each end. Note that competition ≠ domination, despite the availability of Eurostar there are still airlines filling plenty of planes between London and Paris because the well-connected airports over there can be more convenient than battling into the centre of the cities with the commuter crowd.
If Australia was as serious about better transport as European countries are then we would be looking at starting to make the baby steps which would eventually lead to a 250+ km/h alignment between Adelaide and Melbourne - not actually building the high speed track, but easing curves and improving clearances every time any piece of infrastructure is renewed for the purpose of future proofing the route. Before anyone points out that it would not be commercially viable, it's