Absolutely agreed. I want Steam for movies and music. I'd actually start spending money DVDs and TV series if a service like Steam existed. (Well, more money than my current "buy Supernatural on DVD and nothing else" policy, anyway.)
Essentially a Steam for everything would never be able to have DRM otherwise certain devices would get locked out, which returns us to the original problem of DRM. Steam just locked onto a niche market and realized their potential.