HTTP/HTML already is delivering multimedia. It this pig (or car)
don't learn to fly, some other tool that does fly will take
over the exponential growth the Web now enjoys. (Indeed, the
Web's growth curve is largely due to clicking on huge image,
sound and video documents!)
People want to deliver documents, and documents are going
multimedia. Notions of purity or "What HTML was intended
to do" make sense in the abstract but may not be compelling
in every specific case.
You may be right -- PDF, which has precise rendering and
multimedia and hypertext hooks, or a competitor, may be the
way that really elaborate networked multimedia gets delivered.
But then HTML will be rendered (pun intended) a footnote.
/rich