HTML has a lock-in because it's supported by web browsers directly, exactly like JS. I have a profound dislike for anything SGML-related but what can I do? Display my website as a PDF? At best I can transpile an other format into HTML.

HTML and JS have succeeded in spite of their clunkiness, not because of it.

Markdown on the other hand has no such lock-in, mainly because it usually gets transpiled into HTML or other formats. Replacing it is a lot easier and can be done incrementally without client support.