[zbentley]

The business logic many (perhaps most) programmers work on is primarily occupied with gluing together multiple state stores (databases, caches, message queues), running some very simple computations, and writing the output to some IO sink (often a web response).

Sure, you could extract the computations part out. But that barely moves the needle on testability/cleanliness, because most of the business (or business-value-driving) logic is the data flow management--highly stateful IO coordination.