> What would be the advantage of the replacement compared to TeX/LaTeX?

In my experience, LaTeX is pretty clunky, unintegrated and poorly documented. To do any kind of useful typesetting I have to depend on at least five or six third-party packages, and that’s assuming that I don’t need anything ‘fancy’ like nicely formatted references, customised lists or tables, diagrams, hyperlinks or — God forbid — code blocks with syntax highlighting. (Though to be fair that last is a bit difficult in any typesetting software.) If no package exists for something I want to do, then I have to make it myself: a forbidding task, given the complexity of LaTeX’s internals and of the TeX language itself. Besides, TeX is getting old now; it was created when computers had less memory and power, and it shows. A fresher, more coherent typesetting system retaining all the listed features would be clearly better than LaTeX — no mean feat, considering how much better LaTeX is than its main competitors.

(Admittedly, other TeX-based systems do solve some of those problems. LuaLaTeX makes some things a bit easier to accomplish, and I hear that ConTeX is somewhat more featureful out-of-the-box, though last I tried it was nearly impossible to get working on Windows and was documented even more poorly than LaTeX. Either way, both of these still inherit the problems inherent in TeX itself.)

I think it comes down to whether the complexity of TeX/LaTeX is essential or accidental. My feeling is, more than half of the complexity is essential (typesetting is, like human grammar, full of obscure rules and exceptions), and therefore, it simply doesn't pay off to anyone to rewrite the system (not to mention, the more essential complexity is in a given problem, the more accidental complexity you're going to accrue).

There has been unsuccessful attempts to remove the accidental complexity, like Lout. It is very hard. If you limited your requirements to "do not have complete control over typesetting", as I suggested, then lot of essential complexity (of complex typesetting rules) could be converted to accidental complexity (simplified typesetting), and then such a system would be easier to create.

Though such a thing probably already exists in systems like LyX, MS Word, LibreOffice. They can do typesetting with vastly simplified rules (especially of equations).

> I think it comes down to whether the complexity of TeX/LaTeX is essential or accidental.

I tend to agree with this, but I do think quite a bit of the complexity in LaTeX is accidental. For instance, the existence of numerous packages for basic typesetting tasks, none of which entirely work with each other, are a clear example of unnecessary complexity. Also, as a macro-expansion language, the nature of TeX itself makes some things more difficult than they need to be — consider basic arithmetic, to take just one example.

Furthermore, I’ll note that age really is a big factor here: there are many things we want to do today in modern, computer-based typesetting which were much less important back when TeX was invented. For instance, in a recent presentation I wanted to make two images transparent and overlay them; I had to drag in TikZ (quite a bulky package!) to do so much as modify their opacity. Similar examples are numerous, ranging from hyperlinks to vector graphics to comments to animations.

Of course, typesetting is a complex task, and a typesetting program is never going to be simple. But I feel that fixing the above problems would at least make it somewhat easier than it currently is with LaTeX.

> There has been unsuccessful attempts to remove the accidental complexity, like Lout. It is very hard.

I’ve never used Lout, or even heard of it before now; could you expand on where it ran into problems?

On docs, texlive-full (or similar naming in Guix/OpenBSD/Nix...) will provide the PDF documentation and an intro to LaTeX.

As does MiKTeX, which I use. (And it’s all available online in any case.) The problem, as I see it, is more with the documentation of LaTeX and TeX itself: there are a few non-official tutorials and guides, but very little material providing a comprehensive manual of the language and how it works.