A lot of the benefits of Guava have been supplanted by additions to the JDK in JDK 7 and JDK 8 (e.g., the diamond operator, lambdas, and streams). We are no longer shading dependencies in the Elasticsearch jar (see #13252). We do not have a proper Java client today; this means that developers that depend on the Elasticsearch jar are likely going to have to deal with Guava version conflicts (which are likely since Guava is popular). Developers do embed Elasticsearch inside web containers which creates conflicts with the provided versions of Guava. Plugin developers encounter issues with Guava version conflicts (again, because Guava is popular). All else equal, fewer dependencies is a positive. In short, the internal benefits have been reduced, and there are external benefits to developers to removing Guava.
