Fall behind in what sense? Obviously, if the multimedia content isn't transcribed quickly enough, then that makes sense, but I would also guess less consequential than if a given page weren't transformed from 'marked up' to 'text', but in the latter case, if the backend is dynamic, and just changing the output format, why would it fall behind?
People just don't want to maintain 2 sites. Extra features and sections often don't get updated to the text only rendering.
I guess my point is that screen readers like (even prefer) well written markup. A text-only site is a kickback from when they couldn't and the accessibility guidelines were written to deal with that.
Real world experience shows us those sites quickly fall behind. Text-alternatives are necessary for multimedia content, text-only sites as an alternative to good markup aren't.