The wealth didn't need to go anywhere! That's kind of the point. What's confusing is that pre-decline Rome, on the face of it, seems pre-modern in many ways, b

The wealth didn't need to go anywhere! That's kind of the point. What's confusing is that pre-decline Rome, on the face of it, seems pre-modern in many ways, but this is just a simulacrum, because there was none of the productivity increases that underpin the modern world. Example: 19th century London: big in part because there are lots of factories there! 100 AD Rome: big because of years of taxation routing grain there! Yes, there was trade. Yes, cities did do a lot of self rule. Yes, the early government was pretty minimal. See Rostovtzeff for some fun with this. But there wasn't really much self-sustaining about the cities. So back to the original question, there is no "stock" of wealth. The buildings are the results of wealth, not (perhaps baring things like aqueducts) the source of wealth. Without the political control over the many agricultural laborers and their output, the wealth just dissipates. There is a lot more 0-sum stuff going on than the modern world, where conversely the positive-sum nature of things is not well appreciated.