The plant derived medicines come from plants known to indigenous people to have activity. If there's no indigenous knowledge available there's no way to study the plants and make modern medicines.
Of course you can still study plants and make medicines. That's exactly what your indigenous people supposedly did in the past, didn't they?
And if they did it then there's absolutely no reason to think that modern pharma can't produce the same knowledge. And the whole assumption that there is a body of invaluable knowledge somewhere, held by indigenous people but somehow unavailable to modern science is very strange.
