Groovy 3 is full of nice-to-haves. Which is why it's taken forever. Anything that was a "problem" as you say got put into 2.4 / 2.5 (except for solving the Java9 module problem which is unsolved in any release ...).
If Groovy's project managers at Apache don't want a Groovy 3 release to avoid a debacle like Python 3 or Grails 3, then they should just come out and say so. Instead, they're letting others waste their time working on it while putting up roadblock after roadblock to slow it down.