There might not be a lot of market value in doing greenfield JSP development, but being able to navigate legacy codebases and maintain them without breaking critical user flows is a crucial skill. If you only ever use the latest and greatest then definitionally you won't have direct experience working on legacy systems.
If you want to stay up-to-date with new technologies, try to identify places where you can rewrite core subsystems in a safe way. When you pitch this, make sure to explain why it would be useful to the company.If that goes well, you'll have more credibility as you suggest rewriting more of the codebase.