It is a non-trivial problem, but one common enough that I would suggest any Actor library worth its salt would include mechanisms to tackle it. (Here's Akka talking about back-pressure support in Streams, for example. https://doc.akka.io/docs/akka/2.5.11/stream/stream-flows-and...) Akka also provides infrastructure for actor pooling and routing.

I don't see the manifestation of this problem as forfeiting the point. You could on the opposite extreme write one monolithic application that does everything: web interface, application logic, and data storage. It's readily apparent that for most use-cases, the benefits of separating, say, your web and database software will more than outweigh the added complexity cost of running two applications and routing requests between them. (And we don't anymore balk at running pools of web servers in front of our databases.) Continue to chop your application into smaller segments and of course you'll see diminishing returns. The application just has to be complex enough to raise the expectation of benefit from segmentation.