As a side note, TeX engine could support Markdown or HTML or many other syntaxes too: it's easy to redefine control character in TeX to be anything instead or in addition to "\" (I've actually done that with Plain TeX a couple decades earlier to allow two-byte UTF-8 input by making all first-bytes of valid 2-byte UTF-8 sequences into control characters).
So, LaTeX being "unergonomic" is only relative — it's pretty ergonomic compared to things like HTML but especially DocBook or TEI SGML/XML schemas, but less ergonomic than Markdown or even Plain TeX. However, it inherits the most complex part where it is extremely ergonomic from Plain TeX (for the most part): editing math formulae.
But it's also much richer in expressing intent than any of those, and from what I can see, compared to Typst as well — LaTeX is basically semantic markup for excellent printed output (where Plain TeX is excellent printed output with no semantics, DocBook/TEI are pure semantic markup, and HTML/Markdown/Typst are somewhere in the middle too).
So, LaTeX being "unergonomic" is only relative — it's pretty ergonomic compared to things like HTML but especially DocBook or TEI SGML/XML schemas, but less ergonomic than Markdown or even Plain TeX. However, it inherits the most complex part where it is extremely ergonomic from Plain TeX (for the most part): editing math formulae.
But it's also much richer in expressing intent than any of those, and from what I can see, compared to Typst as well — LaTeX is basically semantic markup for excellent printed output (where Plain TeX is excellent printed output with no semantics, DocBook/TEI are pure semantic markup, and HTML/Markdown/Typst are somewhere in the middle too).