Ten year is a long look into the future. If you take the long view, you also need to think about where the rest of your system will be in ten years, will you still have browser based clients? Will you have the same database or whatever store you use. Since Groovy is a JVM language, how will the JVM look in 10 years? If you are concerned with your investment into a legacy codebase, you might want to look into how people are currently dealing with Cobol code on IBM mainframes and similar things - often the old code is still around, but accessed through some kind of service layer. So if you make sure that you have well defined interfaces to your code, you stand a better chance.
Also, how maintainable will your code be in 10 years? I've just finished a major rewrite of a system made for Grails 1.3.5. The original code was not written with I8N in mind, and all the external systems it connected to were changing their interfaces. It was cheaper and more efficient to re-write based on the original spec than to try and refactor the old code.