I still don't understand that why is the system not zero sum. The system has a way of replenishing itself. But if we consume (or harm nature) at a rate higher than it can replenish, then it'll not be able to sustain.

Why do you say that the system is more complicated that zero sum ? Is it because of the infiniteness of sunlight ? Or is it because of the scale at which humans effect nature, or the sheer vastness of nature. Is this from the system perspective, or a human perspective ?