"The second biggest issue is that there isn't a good TeX -> HTML compiler." This is the crux of the problem in using just TeX for writing books: it focuses on producing camera-ready output at the expense of literally everything else.

reStructuredText isn't itself a typesetting language. It's meant to be something of an interposer between the semantic meaning of what you're writing and the thing that does the typesetting. The hope is that it, unlike something heavily constrained like Markdown, gives you the tools to capture that semantic meaning and transform it into something that can be typeset.

Like, the goal isn't to throw out TeX. The goal is to be able to target TeX as a typesetter along with Web browsers and e-book readers and even things like GNU info and troff without having to rewrite the content wholesale to fit each one.