At the time that the first Woodstock happened in 1969, it was a one off, and people treated it as such. They all headed to the New York area expecting to see a one off show, with no idea how many other people had the same idea, or how huge the event would turn out to be. They came for a concert and left having become a part of history.
Because of the success of the original, a whole series of Woodstock events followed (although the organizers eventually managed to work out the whole 'charging for entry' thing), and the fields that it happened on are almost considered to be hallowed ground for musicians and fans of music. Today, the original site hosts the Bethel Woods Center For The Arts, which opens for musical events and performances all year round. We think the people behind the first event would consider that a pretty good legacy.