They state up front that it's not a sustainable system. That's why Sphene needed dimensional fusion.

'I want a world without suffering. A world where nothing is lost... Would that such a place existed. But the Endless only grow in number, and ever more life force is needed to sustain them. Within the barrier, we've managed to curb the cycle of rebirth these few hundred years. But we've reached the limit. That's why I looked to interdimensional fusion. To harvest what we needed from your world. From your people. And with the key, we will have it.'
- Sphene, Through the Gate of Gold, Lv.99.