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> I suspect most economies would hit a saturating point where most people are underemployed and there's no productive work left for new entrants.

I don't know if that's actually true. When you have more people, you likely need more construction to house them, and more restaurants for them to eat at, and more shops for them to shop at, etc. And more delivery drivers, etc. That's not to say a mass of people is enough to form a thriving economy... I think there does need to be some core product or service or some of the people are contributing to or a resource they're harvesting, because the economy either needs to be self sufficient or be able to exchange something of value for the inputs it needs.

It's really hard to measure, of course.