Hi,
I think your English is better than mine.
However, I believe your first reason is not convincing.
Indigenous people do not lack knowledge; on the contrary, they have extensive knowledge of their land and surroundings.
Although they only have little understanding of other modern civilizations, theoretically this does not affect their persistence.
You wrote this:
indigenous people still live widely in natural environment such as in a small islands, caves, rainforest, and little do they have contact with other society.
,which means most indigenous people do not need modern knowledge because they still live in their original environment.
The food is more and more scarcity and a lot of disease threat them everyday. They can not find enough food and also can not treat serious illness. Hence, the number of distinct people decreases quickly.
This phenomenon is actually due to the negative effects from other modern civilizations, not because the aboriginals lack knowledge.
I suggest you develop a new idea.
Good luck