I mean, it's not a joke for its intended purpose; that's fine. It typesets the #$@! out of documents. But TeX is a joke for mathematical notation in particular. Put another way: the best way to understand what an arbitrary mathematical expression in LaTeX really means, is to render it as an image and read that image. TeX can be understood best as a concise way of writing a certain class of vector images, and when you are reading TeX you are reading a computer program which generates an image.

I'm not saying it can't be used for this context, of course it can, I have used it a ton and found it quite enjoyable. The fact that it's an image-based representation makes it very easy to switch from `\int_A dx~\int_B dy~f(x,y)` to `\iint_{A\times B} dx~dy~f(x,y).` However let's not mistake the fact that TeX does not know and does not want to know how you are using the `\int` and `\iint` symbols, is 100% OK with omitting those `~` characters, and has no semantic conception of what `dx` and `dy` are. If TeX were the CAS that it doesn't claim to be, as far as it's concerned that expression could be canonically refactored to `\iint d^2fxy(x,y),` since no one wrapped the `dx` and `dy` in curly braces. TeX claims to be a typesetting and layout system, and it does that well; it's not trying to be a universal mathematical notation.

The contention of the original link posted is, all of these image-based formats like PDF and lecture videos are going away. This may or may not be true, but if it is true then TeX is not going to survive the death of images, precisely because it is a programming language for a class of images.

Maybe something else will survive. The biggest player right now is the Wolfram language, of course, but that can look terribly unwieldy too.

But the point is, mathematics written as a plaintext document needs an interpreter, unless you have been writing papers with math notation for some years. But it is still far more cumbersome (and looks ugly) than writing formulas by hand. And that's not exactly plaintext learning material anymore, then, even though the sources are text files (unlike Word documents).

My ideal workflow (the one I dream about) would be a document camera -like setup that parses math I write on a paper (or a blackboard) into LaTeX (or MathML) style format and ~immediately renders it as beautiful document (with MathJax or similar tool). Like Overleaf, but without typing. (And of course then I could open the source file in text editor to make edits.)