Remember the snowball-effect and ripple-effects of economic collapses - sure, the initial change in actual economical conditions could, from an objective point of view, be quite survivable; but the collapse of several high-profit industries on economicall-important worlds could trigger a chain of events that would do far more damage. For example, think of the major banks financing these key industries; when these industries go bankrupt, it will hit these banks very hard, and might cause them to go bankrupt themselves, or, for the very least, increase the interests rates for their other customers (which will cause a further recession by making investments far more expensive) in order to compensate. Or, investors might panic (a common phenomenon during economical downturns) and start selling corporate stocks en-masse, which might cause the collapse of many stock-exchanges. Or workers fired en-masse from bankrupt industries might revolt in demand for work, or, atleast, means of sustainance. Or all three combined. Or more.
Another fact of the Imperial system is given on Hard Times, p.57: the bigger nobles are the major corp stockholders. So an economic collapse of several key industries might trigger a civil war...
While I agree that the Imperium is somewhat decentralized (due to the long communication times), most planetary economies within its borders had evolved for centuries in an environment where massive-volume interstellar shipment was widely available; thus, planets will specialize alot, especially in the highly-developed core areas, and interstellar economic ties would go very deep. Remove these ties, and massive economical damage results.
