I'm afraid you rather missed the point in the last paragraph -- "we need more such systems with which to create and render content, before denting the DOM+CSS hegemony". Fixed format is much preferable for textbooks or other publications which have many diagrams as part of the body text.
Textbooks, sure - other publications with many diagrams - that's my whole point, no (the fixed format often makes understanding worse because they can't just put the diagram in the specific spot that refers to it, and finding that spot is additional work) - also pdfs only really support fixed, non-interactive diagrams.
