Here’s a thought experiment. Let’s imagine that Apple decided to combine their engineering resources to form app teams that delivered both iOS and macOS versions of applications.
In such a scenario it may seem logical to retain application features common to both platforms and to remove those that were perceived to require extra resources. Certainly Automation would be something examined in that regard, and the idea might be posited that: “App Extensions are equivalent to, or could be a replacement for, User Automation in macOS.” And by User Automation, I’m referring to Apple Event scripting, Automator, Services, the UNIX command line utilities, etc.
Let’s examine the validity of that conjecture, beginning with overviews of App Extensions and User Automation.