You answered your own question.
Some, or many marsupials
found their way to Australia.
The others did not or did not survive
the trip or the climate at the time.
That is a completely preposterous claim. How did the marsupial moles outrun the gazelles all the way to Australia? Even more, we don't find fossils of placental mammals in Australia. Just a coincidence?
It's fun to make up stories based on Science-y facts.
It is called Science-Fiction.
That is exactly what you are doing.
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