My point was that trade, in a wide sense, creates wealth. No one would cross the sea or the desert if there wasn't a prize, so I take for granted that there is an inbalance.

"Bottleneck" seems to suggest that commerce is like a natural process. It isn't. There are trade cultures like Phoenicians or Renaissance Venice, but often the powerful don't like commerce. Commerce implies some values that subvert authoritarian regimes, maybe just because different ways of understanding life make contact.

So when I hear bottleneck, I think of trade restrictions, not flow. You mean bottleneck as a funnel where the flow is more intense and thus more susceptible to capture wealth. It makes sense, but it's somehow a twisted metaphor, because it's the inbalance (gravity, or demand + the cheaper production elsewhere) that creates the flow, the bottleneck just make it easier to exploit. But a cascade can be wide or narrow, tall or short. In any case, there's water falling...