Regardless of whether or not 20TB is hording / excessive / inefficient, what it almost certainly is is replaceable. Let's face it, you aren't CERN, most of you data is probably media that you can reacquire with relative ease. It's not being stored because it's irreplaceable it's being stored because it's convenient.
There's a large, flawed, assumption running through this thread that it's "easy" to reacquire all media.
Between torrents and usenet, trying to find good rips of content that's over 6-12 months old is often impossible. Good luck trying to find any older show that's not sci-fi or super popular. Often studios don't sell DVDs or Blurays of these shows anymore, if they even did to start with.