I do not see the need to constantly evolve a language and/or push for rewrites from scratch. Java as a language succeeded precisely because it was relatively simple, yet powerful enough. For example, I believe that adding generics was a mistake, especially in a crippled form (due to compatibility with previous language versions).

I favor switching the language/environment once you feel that you need more expressing power. E.g. use Scala, Jython or Clojure if you need JVM, or perhaps Erlang for server-side.