> While HTML and CSS started in the ways that you've described. That's not what they are for now.
Has HTML eveolvd beyond a single tree? No. Has HTML evolved beyond displaying (barely) just text and images? Also, no [1]. Have HTML and CSS evolved to a point where it doesn't involve a reflow/repaint of the entire page or large chunks of a page to do the simplest things? Also, no. Have there been significant updates to the DOM model? No.
> What use does framing HTML and CSS from the perspective of their origins have?
It helps you understand what the are, not what people pretend they are. It also helps you understand why things are they way they are, why so many things are complex, or impossible, or interact in unintuitive ways with each other, or why a 10-page essay on how to center an element ends up with a yet another "gentle flex" hack reminiscent of the "Search for the Holy Grail Layout" of 10 years ago.
Because no matter how many times you use "holistic" and "unison" in your speeches, it doesn't change what HTML + CSS are essentially are at their very core that permeates everything: a way to display (rather poorly) static content that consists of text and a few images, in as few render passes as possible (because it was developed for the 90s era computers). Everything else is a series of gargantuan efforts costing millions of dollars to try and mold them into something they are not.
[1] There is a reason why most UI frameworks for the web (doesn't matter whether it's pure CSS or it's Javascript through and through) implement the same half-a-dozen to a dozen components: a button, a label, a badge, breadcrumbs, a text input. Very few dare to implement a date picker. Even fewer go for complex elements (actually working accessible modal dialogs, virtual lists, tree views etc.). Because the moment you step outside the "text + images" capabilities of HTML, you're screwed. And no amount of hacks added to CSS can help you with that. I won't even mention the impossibility of high-performance animations. Sure, you can CSS-transform something... as long as the original element stays in place and never moves because actually doing proper stuff in "CSS and HTML that's not the same now" is just as expensive and nearly impossible as it ever was [1.1]
[1.1] https://css-tricks.com/using-css-transitions-auto-dimensions...