I think that, as with standards, the problem isn't no replacement but too many: see Pandoc (http://johnmacfarlane.net/pandoc), Lout (http://savannah.nongnu.org/projects/lout), its (not yet existing) successor Nonpareil (http://sydney.edu.au/engineering/it/~jeff/nonpareil), Skribilo (http://www.nongnu.org/skribilo), and Pantoline (which I can't find with a quick Google search), for example. The problem is that TeX has so much inertia, and such an eco-system behind it, that getting significant investment in any one of them is likely to be difficult to impossible.

I think compilation to TeX is probably always going to be less successful than compilation from TeX—that is, taking the same code and (slightly) improving the processing. See the pdfTeX (http://www.tug.org/applications/pdftex) and LuaTeX (http://www.luatex.org) projects.

As a remedy to too much ambition even when trying to build directly on TeX, see the fate of LaTeX3 (http://tex.stackexchange.com/questions/953/why-is-latex3-tak...).