Well, you can drive the same model of automobile for 15 years, but this doesn't mean the manufacturer of the model of said car needs to either stop making the model or simply make one new model every 15 years. Both options are unrealistic. VS is the container for all of our core platform development tools. As such, it will continue to evolve as a whole (a package of next generation development tools and experience).
Visual Studio, like Windows, needs to evolve to meet new demands and continue to serve as a powerful IDE and integrated toolset for software engineers. Debuggers evolve. Runtimes evolve. Programming languages and compilers evolve. Libraries evolve. Editor environments evolve.