The goal is to preserve one copy of every published work. The universe of unique titles has been estimated at close to one hundred million items. Many of these are rare or unique, so we do not expect most of these to come to the Internet Archive; they will instead remain in their current libraries. But the opportunity to preserve over ten million items is possible, so we have designed a system that will expand to this level. Ten million books is approximately the size of a world-class university library or public library, so we see this as a worthwhile goal. If we are successful, then this set of cultural materials will last for centuries and could be beneficial in ways that we cannot predict.
Even "centuries", of course, is not very long. But it's far, far better than doing nothing at all.