Because Polymer observers cannot track arbitrary fields of an object, but they instead track a property "path" (object.subobject.property), the dialog can't cleanly add a property to each sink that indicates launch state and have Polymer observers track them.  Instead, maps similar to the current sinkMap_ and routeMap_ would have to be added to track multiple launching sinks, multiple sinks with pending routes, etc.  This adds complexity to many parts of the UI.  Since there are no clear use cases for this now, I'm marking this as WontFix and we can reopen this or use a new bug if this becomes more important for a concrete reason in the future.