Important to remember that things like React or Java, they aren't ever finished, they are constantly being worked on and improved on, so while there is a central brain trust (React has the React team at FB, Java has the Java Executive Committee comprised of devs from Oracle and other big companies like IBM, Twitter, others. these devs are eventually going to retire and will need to be replaced by community experts) these open source communities are still very much dependent on dev community assistance/feedback for the successes of these languages/frameworks to keep the gears churning.