Without trade, those places wouldn't have existed, let alone become wealthy and grand. Trade was a huge source of wealth for these cities because they could tax it through import and export duties or port fees. But the fact that they are no longer powerful shows that trade can be a pretty weak foundation upon which to build a polity, even a small one. There are many reasons for this, like high taxes can motivate traders to find other routes, for instance. But the main one is this. Reliance upon trade makes you especially vulnerable to the peaks and troughs in the global economy. The legacy of the merchant kingdom in Southeast Asia is still alive and well in Singapore, for instance.