While it is true, that Markdown is available in more places, it is also true, that it as well "requires custom tools". Just that those tools already got put in

While it is true, that Markdown is available in more places, it is also true, that it as well "requires custom tools". Just that those tools already got put in place by people developing things. There is no technical reason keeping developers from supporting other languages as well. It's more of a social issue, that they don't. I also doubt, that Markdown will be able to be extended with the same "ease" HTML has been extended. Markdown does not have a systematic syntax like HTML does. It is not parser and machine friendly. It may be human friendly, but that doesn't make it easy to extend. In HTML on the other hand, one could, theoretically, just come up with a new tag name and introduce a new tag. Mind, we are talking about using both formats for writing a document. Of course Markdown doesn't have the same burden of already being the workhorse of the web and being directly interpreted by browser rendering engines. That however is an aspect that resides outside of the actual format considerations. We already got tons of Markdown dialects and most of them don't gain particularly much traction, because they are only supported in niche tools. The only one of them that could be considered standard is maybe common mark, with its extensive test suite. Specialized ones like Pandoc Markdown for writing papers exist, but will most likely never leave their niche. What I could imagine is, that maybe the changes from dialect to dialect could be outsourced into "extensions" and then one would need to pick "syntax conflict free" extensions. But since the format itself is kind of ad-hoc, and not systematic like HTML with its tags, it seems also unlikely, that such an extension system would work well.