Well, some event organizers will keep an archive somewhere or save some presentations on a CD that you can order online. Not all is lost.
Yet, you still need to go through them, locate the presentation in question, pull out your credit card and place an order. At the end of the day, somebody else still gets to decide what you get to see or download.
You wish you could have instant access to all your past events, to replay them anytime you want, as many times you want.