You had me at “structural markup”…
Have you thought about the evolution of publishing recently? Here’s someone who has:
Typewriting (like most handwriting) is a process of applying ink to paper and making the text readable — in more or less one operation (by pressing down keys, letters are punched on to the paper). It probably makes sense to say that you must write before you can read, but clearly the storing and making a representation of the text are performed in one inseparable operation; by applying ink on paper.
In digital writing, by comparison, writing is performed by the execution of a series of discrete steps. By touching the key of a keyboard, signals are sent to the computer. Here the signaled information is converted and handled by the central processing unit and temporarily stored in main memory. In the computer, new signals are created and transported to the display unit. On screen the text is represented in a visual, readable way. In this digital cycle storing and making a representation of the text are performed in two different operations.
The article, Digital Text Cycles: From Medieval Manuscripts to Modern Markup (January 2017: Updated to fix broken link) is long, but worthwhile except for an odd digression:
The belief that a text can be rearranged and moulded by technological means, presupposes that content and presentation can be treated independently, as logically distinct features: it rests on the false supposition that any kind of written or verbal content can be presented at will in any medium and for whatever purpose.
My approach to single sourcing is much more nuanced. Certain types of information are well-suited to reuse and changes in presentation. For example, glossary entries could be presented in alphabetical order at the back of a book. In online help, it makes more sense to create pop-up links from the term to the definition. Both presentations can easily be generated from a single glossary source file.
But it’s a long way from my glossary example to the assumption that you can present any content in any medium.