Before you Delete a Feature Service, a warning box should pop up with a list of dependent Web Maps, so that you don't inadvertently delete something that's being used; or so that you can systematically delete those Web Maps as well. [As we create more and more maps, it's get harder and harder to keep track of what's dependent on what].
This is related to jesterjace's Idea, "Processing ArcGIS Online Service Updates More Effectively" (the ArcGIS Online proposal).