You can have a steady-state economy even under the most classical of economics like Solow, simply when capital stock stays constant because investment equals depreciation.

And it's no coincidence that Saito is Japanese because that's what Japan has been living in. They've not grown in decades. Without overlords, it's an adaption to a world where the average citizen is 50, ecology cannot sustain consumption and significantly more time will be dedicated to mending the environment, taking care of old people, and putting technology to work to make that possible.