For two reasons:
First, if you know right now that you or your customers can live without a feature for the next five years, then you don’t need it at all and you shouldn’t bother building it.
Second, The 67th-most important-thing you put at the end of your list today will never, ever make it to number one. The most important thing you’re going to spend your time on five years from now isn’t even on your radar yet.
Working from a 5-year roadmap gives us a false sense of control and creates organizational inertia against aligning to meet the real challenges of the day as they emerge.
Instead, invest your time learning more about your customers’ current needs and building the muscles to be responsive to them right now. Not five years from now.