Ecosystems have _finite capacity_ to cope with what we throw at them (and still be useful to us in the broadest sense).
The qualification in parens is just there to take care of the objection that no matter how hard we try, we won't be able to eradicate all life on earth with current technology. But we can make life pretty miserable for humans with current technology.
The benefit to us doesn't have to be in any monetary sense. Just the satisfaction of knowing there's a beautiful forest would count.
