I guess I really don't care that much about the various audio or video formats, and also don't have much confidence in the longevity of any of those. Then I am so old that my experience stretches from the old mono LPs and reels of 8mm film to what we have today. As far as ebooks go the only issue is the DRM, and that I always remove as soon as I obtain the book (with the additional step of converting to epub if not already in that format). As others have noted the epub is just a compressed set of text files with html mark-up so the content will always be there. Also pdf has been around for so long and there is so much out there in that content, not just books but scientific articles, manual, etc., that I figure it will remain useful for my lifetime at least.