Unix is still the same (in some ways) as before because Unix had a remarkably simple mental model, and the ways we try to replace them don't. Unix didn't last forever because it had features, good god! No, it's because it was so robust and flexible as to serve even a modern system. TeX, and Unix, and all these things, were hackable. (If you don't agree about TeX, read the TeXbook and some traditional TeX, not this LaTeX crap that's grown around it.) That doesn't mean it these systems are irreplaceable. (Hell, I'm working on a project to replace the Unix shell...) It just means you need to put more effort into devising a very good mental model. Something that actually captures all of the variation you want. So, don't replace TeX because you don't like TeX. Replace TeX because you have something better, something obviously better, something that you could explain to a five-year-old.