>But there are no slaves or spices, and no gold or other metals at prices that make the economics even remotely sane.

Very much untrue, at least in part. The solar system, including the asteroids, moon and other bodies, is absolutely, insanely loaded with valuable minerals and materials to a degree that makes what the old colonial powers sought in the New World and Africa pale in comparison, it's just extremely difficult and expensive to reach, for now, so on the second part of your comment, you're correct.

However, it's worth noting that the technology for extraction absolutely would scale down in cost over time to the point of space mining being as routinely profitable and practical as mining on earth.

I don't like comparing space exploration to the fundamentally cruel and exploitative schemes of Earth's governments during the colonial periods but even they at first had to deal with high costs and little benefit, until they developed scale. Humanity could do the same thing beyond the earth, and without all the grotesque moral failures of destroying whole human cultures in the process.