I pretty much agree with everything you've said. LaTeX is really terrible conveying error messages, and it's very difficult to get stuff right if your way is not LaTeX's way.
Given the humongous amount of plugins and related developments (like Knitr, which I love) I don't see a replacement coming soon. Some simpler markups like markdown or org-mode can go part of the way, covering simple articles and even whole books; I wouldn't write a book now in LaTeX if I can avoid it. But for some niches there's nothing that beats LaTeX.
Besides, there's some stuff there that's pretty interesting: XeLaTeX, for instance, biber as a replacement for bibtex... I think that eventually a better LaTeX will simply be... a better LaTeX.
This is an issue that arises from time to time, and I seem to remember to have seen some articles that even provide PoC for replacements. None of them got to fly. So I guess it's LaTeX (or LaTeX++) for the foreseeable future.
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