(La)TeX is a bad fit because its document model is based on paginated documents with a fixed page size, whereas HTML documents are intended for variable viewport size. LaTeX is to HTML as PDF is to EPUB.

No it is not.

LaTeX can, whether through the old model of dvi, or modern tools such as xlatex and pandoc, directly produce numerous document formats or "endpoints" as I consider them, including HTML, ePub, plain ASCII (or UTF-8) text, or paginated formats including ps, PDF, and djvu. LaTeX is not itself fundamentally print-oriented. The fact that it can and does produce excellent print-formatted output is a feature, not a bug.

What it is, and pointedly in ways that HTML lacks, is capable of intrinsically handling document-centric (not merely "print") elements including footnotes, endnotes, and formulae, all of which still require kludges after over a quarter century on the Web.

Markdown itself does not address several typographic or document conventions, including formulae, but also odd omissions such as underline and coloured text. Whether those get shimmed into Markdown, or an alternate (light- or heavy-weight) markup language is adopted, isn't clear, but those are very annoying lapses.

For the vast majority of documents, this does not matter. Most online content, say news media, use little more than paragraph, italic, and anchor elements. Even bold and list are rarely used. Authoring in Markdown should be almost wholly sufficient, but it's (La)TeX which has sufficient richness of expression to serve as the common underlying document format language.

Late edit: It also occurs that another principle angle of attack on HTMK alternatives, raised elsewhere in this thread, is that these cannot guarantee pixel-perfect presentation. That results in rather a "damned if you do, damned if you don't" situation: propopsed markup alternatives either cannot guarantee layout or over-guarantee layout. These objections rather want for consistency.