In the end, it's all about sun-provided energy input. Coins or treasury, or freshly plundered lands really represent the means by which to convert energy into g

In the end, it's all about sun-provided energy input. Coins or treasury, or freshly plundered lands really represent the means by which to convert energy into goods that can be consumed whether that's food. But you can't get more slaves if you don't have more lands, but more lands means administration difficulty increases, which means you need more soldiers and bureaucrats, who are not directly producing food.