Java is what it is at this point. Tools like Groovy give developers the chance to get a gentle introduction to new techniques such as Functional Programming, Parallel Computing, Domain Specific Langauges, Runtime Metaprogramming, and Compiletime Metaprogramming, as well as a whole host of other techniques and technologies.
The beauty of Groovy as a programming language is how accessible it makes all these advanced techniques to a beginner and how readily it will get out of the way of an expert. This is a beautiful balance of the elegant and the ugly that few other languages manage to pull off well.
Do I think Groovy will supplant Java on the JVM? No, not really. Do I think any other language will make that claim? I seriously doubt it. Do I think Java needs a successor? No. No I do not.
The closest ecosystems that we can draw analogies to for the modern JVM are the .Net ecosystem and the C/C++ ecosystem. The C/C++ ecosystem is probably a better environment to draw lessons from since C/C++ long ago shed its lock-in with a single vendor (that being AT&T long long ago).
I think Java is growing up the same way the C/C++ ecosystem did and C hasn't really changed much over the years. I think Java needs to become the same kind of solid foundation. If other languages come and go on the JVM that needs to be groovy with everyone.
I don't steer any ships in the Groovy navy, I don't direct any plans or strategies, so what I say here is just what I say, and what I say is:
Groovy allows rapid access to advanced computing concepts with a minimum extraneous cognitive load.This is the Groovy language's strength. It is immediately accessible to anyone with any programming background. This accessibility is the vital engine for the success we are seeing in Groovy adoption. It is also the weakness of the language. It means that Groovy must necessarily avoid the traps of becoming too "beautiful" of a language forcing people into certain forms steepening its learning curve too sharply.
That's my opinion anyway. I happen to be one those kinds of guys who likes what I can do with a language. I tend to not get hung up on whether it is pretty, or if it is pure. It's all groovy man.