From my perspective, the reason to have a language for static documents is parsability. With HTML5+CSS+JS, there are few organizations at the scale which can sustain making something like a web browser, a general-purpose web crawler, a11y tools, or similar.

Could it be a common language? Perhaps. I liked HTML2. I hated basically everything which went into HTML3 and HTML4. HTML5 has made a lot of good progress on rolling this back, but we're not there yet. It's possible to write parsable HTML5, but most organizations don't do it, and the ones which do don't have common ways to do it.

It's hard to remember, but back in the HTML2 days, the web had a certain type of client-side programmatic experimentation which is absent today. Anyone could write a tool which would grab things from the web and do meaningful things with them, and many people did. Google evolved from Altavista, which was a tech demo at DEC.

To be clear: The web is a lot more programmatic today in a general sense (client-side browser extensions, AJAX APIs, embedding webkit, etc. etc. etc.), but one part of that flexibility is gone, and it was important.