I believe the effect of that one simple addition would amplify their effectiveness (or at least reach) by an order of magnitude.
After the massive work that their authors put in, the effort of one good read-through would be marginal. If the above surmise is true, you'd be in power law territory.
But you'd need a tool to make such an additional channel as trivial as doing one good read-through.
I'm a little surprised that Bret Victor doesn't do this for his pieces, considering his emphasis on the use of more human capabilities[0].
Implementing the read-back part is trivial [1], and available on all "modern" browsers.
Trickier is making the product that lets authors add this channel without manual tagging. But even this is becoming trivial with speech-to-text API's.
Anyway, I wholly agree with the OP that the first focus of documents in dynamic media should be improved exposition and, by extension argumentation.
[0] "The Humane Representation of Thought" https://vimeo.com/115154289
[1] For example, see a crude implementation at https://willshake.net/plays/Ado/1.1 (click a play arrow in the blue column.) The tracking code is at https://willshake.net/about/text_tracking