The strength of the cultivators from the Immortal Origin Holy Land was too powerful, and Earth was too small, its population too dense. The strength of the humans on Earth was also too weak. If the cultivators from the Immortal Origin Holy Land fought amongst themselves, even minor repercussions could lead to the widespread death of Earth's inhabitants.
Especially after Lin Yun left, with powerhouses from the Immortal World and Heavenly Realm having only just sent people down, it was the time to vie for control over Earth's benefits. These kinds of battles happened frequently.
Over the past decade or so, only several hundred million had perished, and that was under restrained circumstances. Otherwise, destroying Earth wouldn't have been too difficult for them.
But ultimately, several hundred million lives had indeed perished.
Being weak was the original sin, but it shouldn't deserve such a death sentence.