I'm curious what makes this superior to Beanshell. Or any other style of REPL that you could do on the JVM.

Not against the idea, per se, but it seems hardly new ground. And unlikely to be nearly as powerful as a REPL in CL. (Though, again, few things are. Not sure that any are, to be honest.)

Well, Beanshell is dead, last update was on 2005.

Then there is a big advantage on having it as standard tool, instead of something done by third parties.

My question in that vein is more of "why will this succeed, where beanshell failed?"

That is, I had REPL style environments for java a long time ago. And literally nobody used it. I can see arguments for having the REPL being in actual Java instead of a shell subset. But, Java has a long way to go from bootstrapping something in a repl and automatically saving it to something that will work as a normal entry point. (Though, again, even JRebel has existed for a long time now.)