I think it's a reasonable take after 15 years of seeing CSS/HTML/JS being abused for everything and more (when all you have is a hammer…), with the result being that many simple static pages are nowadays monstrous JS apps. My conviction is that, had those technologies stayed simple and focused on delivering the "Document" use-case, turning everything into an app would be a much bigger bridge to cross.
That's essentially where the web was at before the HTML5 days: you could already go as fancy as you wanted, at the cost of pushing your users into a java/flash applet (and through longer loading times, high CPU/Memory requirements, …), and that clearly wasn't the norm.
I would argue that a good layout engine for pages could see good use. See Gemini/Gopher—people like minimalism, but having to use a separate browser makes it probably too niche.
Contrary to what was stated in the suggestion, it’s hardly a solved problem—improvements are being made at a steady pace (grids or { text-wrap: pretty } come to mind)—but it is interesting to imagine, especially if there were extra compelling reasons for engineers to restrict themselves to purer hypertext document API if possible (for example, it could be much more stable, while the app part could be unlocked to evolve more quickly but would be more demanding to developers keeping their webapps up-to-date; search engines could prioritize it; there could be hosting services specializing in it; and so on).
One counter-argument is that you can’t neatly separate the two and engineers will definitely want to use both; one way around it I can see is if hypertext document functionality was possible to use from within the webapp somehow, but I haven’t given it that much thought obviously.
