In my experience the senior title is directly related to see problems in a broader scope or total scope. If you're given a sufficiently scoped task then jr/sr should be able to complete it with similar efforts.
Often times tasks are poorly scoped or insufficient research has been done on it. This is where the senior engineer brings his experience in to play to be able to complete it.
To not become irrelevant you need to be familiar not only with the language, but best practices, and external impact and integration of your changes across all levels. Being able to address issues before the QA stage is critical. So it's not just fulfilling the story it's understanding what the story intended to do and possibly revising it.
