Having tried both TeXmacs and Typst¹, it’s easy for me to understand why Typst is rapidly gaining adoption and why, after — how long, over a decade? — essentially nobody uses TeXmacs.
Typora & Obsidian apply the same ideas to Markdown.
I do think WYSIWYG is an absolutely broken paradigm for anything outside of literally just desktop publishing --- but WYSIWYM has a lot of merit.
Enough that I've been working on a stand-alone Markdown editor component that takes ideas from Typora et al.
J. van der Hoeven. GNU TeXmacs: a free, structured, wysiwyg and technical text editor. In Daniel Filipo, editor, Le document au XXI-ième siècle, volume 39–40, pages 39–50. Metz, 14–17 mai 2001. Actes du congrès GUTenberg.
Reproduced at https://www.texmacs.org/Data/TeXmacs.pdf
For why so few people use TeXmacs, it may be that it is because few people believe that it is worth trying, as one would not expect that it works so well :-)
It's because they gave it a terrible name that strongly implies it's some kind of Emacs TeX editor.
Ooo I thought of another example of good WYSIWYG - Qt Creator's form editor. The only good GUI form editor I've used. Again the editing experience doesn't exactly match the output (e.g. some layout features are visible), but it's clearly WYSIWYG. It just doesn't suck like MS Word.
"corpus-scale conversion work aimed at 90% error-free HTML (currently 75%)"
although there may be issues that I do not understand or did not see (I looked at the paper very quickly) that make it more difficult for the authors than for the simplest possible translation.