> TeX is also not a programming language.

I wrote some macros in TeX, over 100 of them, and in them used programming language constructs including if-then-else, do-while, dynamically allocated typed variables, and file reading and writing. Sounds like a "programming language" to me.

Sorry, TeX was not written primarily for programmers or computer scientists. Instead the target audience was people who use a lot of mathematical notation. In my opinion, the main glory of TeX is how it helps position the mathematical symbols in mathematical expressions, including some really complicated ones.

I know a guy, a good mathematician, who had a really tough time understanding the purpose of TeX. He kept evaluating TeX in terms of what it did for the future of word processing taken very generally, maybe all the way to video as in some Hollywood movies. E.g., TeX is not promising for generating video of a Darth Vader light saber battle. Then he noticed that TeX is not really that future. I finally explained to him that TeX was not trying to be the future of some generalized word processing, thus, was not looking ahead, and instead was looking back and at something he knew well -- the literature of advanced math as in math journals such as published by the AMS (American Mathematical Society or some such). So, TeX was to ease the word processing needed for pages of mathematics as in the math journals and textbooks.

That friend kept asking me to write a converter that would convert a file of TeX to a file of HTML. I kept telling him that such a converter was impossible because TeX was a programming language and HTML was not. I did explain that at least in principle could write a converter to convert TeX output, that is, a DVI (device independent) file to HTML. There are converters, heavily used, to convert DVI to PDF (portable document format or some such).

I like TeX; it is one of my favorite things, and I use it for all my higher quality word processing, the core, original math for my startup, business cards, even business letters. My last published paper (in some mathematical statistics) was in TeX, and using TeX was liberating because I could just go ahead and do the math and not worry about how I was going to get the word processing done, did not have to bend the math and reduce the content to make the word processing easier.

Future of TeX? The fraction of the population that wants to typeset complicated math expressions seems to be tiny, and there are lots of alternatives for others. So, my guess is that TeX will be like, say, a violin -- won't change much in hundreds of years.

The OP mentioned LaTeX: For people new to TeX, no, you don't have to learn LaTeX. The approach of LaTeX is different. In an analogy, LaTeX wants you to state if you are building a bicycle, motorcycle, car, truck, boat, or airplane, and then lots of lower level details are handled for you. With TeX, never decide what vehicle type are building and, instead, work with the parts and pieces -- yes, with a lot of help.

There is Knuth's book on TeX, The TeXbook, and also the books on LaTeX. In comparison, Knuth's book is a lot shorter than the books on LaTeX. So, I got the books on LaTeX, looked at them, and decided that it was easier just to stay with TeX and the macros I could write for TeX. So, for people new to TeX, don't really have to get and read the books on LaTeX.

Getting math typeset was a big problem. TeX is a good solution. Problem solved. We can move on!

The collection of TeX macros I wrote has over 100 macros