It seems to me that there's one very obvious difference: once software is created, creating more copies of it is almost free. The reason pyramid schemes collapse is that they require an exponentially increasing amount of new deposits to keep up with the payments owed to existing depositors, and that's just not how software publishing works. So long as there's a sufficient stream of customers buying, it doesn't cost much more to keep everyone up to date than it does just the new customers, and if there isn't then forcing repurchases won't necessarily fix that.