Struts 1.x does not require defining action mapping to obtain an ActionForward, form beans always have been optional, controller settings are rarely changed. I suggest to create default struts-config.xml with sensible settings for controller and resources, so that simple projects could be started without creating an explicit struts-config.xml file.
Of course, struts-config.xml will be customized more often than chain-config.xml, but the point is to be able to create a simple Struts application without getting into details of what struts-config.xml is for and how it works. This will simplify the learning process.
ActionServlet should not throw UnavailableException if \WEB-INF\struts-config.xml or a web.xml parameter is not found. Instead it should use a default version from the classpath just like it uses chain-config.xml.