Myrtone does not live in Sydney.
Why should we fix something that, unless you're on sector 1 in peak, works reasonably well?
I have been there, and it's not like I live on the other side of the world.
The red train pictured above is one of Deutsche Bahn's Regional-Express sets. Germany, like many other European countries, uses low-level platforms, and from this practice so does America. High-level platforms are very much an English affectation, which is of course why we do it too.
Mind you, yes high platforms orginated in the British Isles, and we did copy them, but that was before double decker trains. High platforms are not uniquie to the British Commonwealth and ex-commonwelth countries, they are common on metro systems around the world, but these all use single deckers.
Because of this, when double-deck stock is used to Australian loading gauge, the lower deck of the stock actually sits below platform level, meaning the intermediate vestibule is the only place for the doors. While low-level platforms are well-suited to double-deck operations, they're terrible for single level stock because you need to have steps inside the car to get up above floor height, which greatly reduces the amount of vestibule space (or, you apply the solution the Swiss used for their upgraded RBDe 560 sets - a dropped centre section with steps up to end-carriage seating areas).
But single decked trains have been phased out of Sydney's suburban railway network, and most interurban services also use double deckers. As for low platforms being more suitable for double than single decked operations, one soluiton is to have separate platforms for trains with different entrance heights, not uncommon in Europe, where platform heights vary between different European countries. Another one, of which I know of no real world examples, is to have doors both on the intermediate and the lower deck, which would allow trains to use platforms of both height. This is similar to a concept devised by someone else who posts on this site:
As for the countries where Double decker trains have doors on the lower deck, have those countries always had lower platforms or did they have high platforms in the past? Suppose it depends on which country, some places have changed platform heights.
Double decker trains in Sydney have been critisised for longer dwell times, and the small area on the same level as the doors may contribute, lower platforms and doors on the lower deck mean that a larger floor area and more seats are on the same level as the doors. I'm not saying that double deckers are a bad idea, they have advantages in terms of length, and are quite impressive, just that it's better to have at lest half the floor area and at least half the number of seats on the same level as the doors.