Imagine web had actual support for hypertext. Then you couldn't just "hotlink" images, but also "hotlink" (i.e. natively quote, and annotate) textual content, with arbitrary start and end-points. As it is, html can't can't even express the equivalent of margin notes or footnotes, because it has some braindamaged ideas about inline vs block elements and the former can't contain the latter (they changed the nomenclature, but I'm pretty sure the limitation remains).
SGML, the meta-language on which HTML is based, has entities ie text expansion variables bound to the content of external resources accessed via URLs or file names. So HTML itself doesn't need such mechanisms if it's understood that general markup facilities of SGML are available (which however isn't the case with browsers). XML (XInclude) attempted to establish more granular inclusion of external document fragments via XPath, but failed on the web. Even if HTML had transclusion, what would the use case look like? HTML as a markup vocabulary was introduced for casual academic publishing at CERN, reusing folklore SGML markup elements that were already widely used at the time. You can argue that HTML has been effectively stagnant while everything around it (CSS and JS) was changed ad absurdum to cater for HTML's limitations, but still in an academic setting, you'll use inline citations or block quotes with references to sources, just as HTML is providing.
You can do all this with the mostly ignored part of Fielding's REST, HATEOAS.
Xanadu has a lot of interesting ideas and Nelson is a visionary, though the idea to visually link works just doesn't work. However, transclusion is an important idea, that is critical for Wikipedia's style of evolution, and will be important as Wikidata and other linked data (semantic web) applications get off the ground.
However, the vast majority of the web consists of cheap short cut web sites that aren't much different than dial up systems. AKA start up money grabbing culture trying to carve up every little idea into a unicorn. And once they start carving things up, they sure don't want to collapse it and help create the semantic web, except privately behind the scenes, in creepy surveillance capitalist and other centralizing projects.
