20-Year Prospect
Believe it or not, the Secret Service hasn't always been responsible for protecting the American leader. They've only existed since they end of the Civil War, and for the first forty years or so they mostly dealt with money fraud (source).
In 1901, after we lost our third president in five decades to an assassin's bullet, people realized that we probably needed a dedicated agency to make sure that killing the president wasn't such an easy thing to do.
Ever since then, the Secret Service has been synonymous with presidential protection. The reason we point out the past is to point out the future; as long as we have a leader who can be harmed, we'll have agents making sure that doesn't happen.
Improvements in technology, data gathering, and detection will all help the task, but the job will remain unchanged—making sure the commander-in-chief doesn't bite the big pretzel, so to speak.