What? Where did they say that in that talk? They simply showed a way to execute native code with V8, so it's easier to run node on nashorn. Stop spreading misinformation.
There is no where they mentioned "replace nashorn", the main feature of nashorn is to interoperate with Java, V8 cannot do that. If you follow the development of Nashorn, they are working on ES6 features right now.
Also replace nashorn with V8 simply is a false statement, V8 is of it's own project established before Nashorn, and it doesn't even belong to Oracle, how can one replace a new project with an old one?
I prefer it's very cut and dry boundaries between Java and JS to Nashorn which tries to help by bridging Java into JS (but complicates both in the process)