[zbentley]

If your concerns are well separated in a monolith (in practice this means being able to call a given piece of functionality with high confidence that it will only talk to the data stores/external resources that you expect it will), adding new entry points is very easy.

Now, it's not trivial--going from, say, a do-everything webserver host to a separation of route-family-specific web servers, background job servers, report processing hosts, and cron job runners does require work no matter how you slice it--but it's a more or less a mechanical or "plumbing" problem if you start from a monolithic codebase that is already well behaved. Modularity is one significant part of said good behavior.