I do not mean to be pedantic, but this is topic I love.

Marsupials do not fill a niche by virtue of their lack of placement. Instead, they have survived so long by virtue of their isolation.

It turns out that the adaptions required for marsupials to birth and raise young without a placenta make them inferior to placental mammals in almost every scenario. They get out competed and die off in almost every instance. South America had marsupials, not placentals, until it formed a land bridge with North America. What happened then? All the marsupials died off with the weird exception of the American possum. The placentals straight up out competed them across the board.

Australia has kept marsupials only because of its extreme isolation. When any type of placental mammal has been introduced to Australia, it has ruined the ecosystem and taken over the niche it fills.

Independent of humans, marsupials are a dying design. We just happen to live at a time when we can see that extinction in process. Yes, humans have sped it up by more rapidly introducing placental species, but we can see how it happened without human intervention as well.