Wouldn’t it be easier for the user if a particular site specified what extensions (if any) they support? And then provide options to enable/disable them? It feels extremely clunky to have to mark up the text just to use a syntax if it’s already supported, probably best to leave activation of the syntax to the site itself. If another site also allows activation of that particular syntax, there should not be any conflicts/portability issues since it is the same specification being used. And why would there be a need to include syntaxes on sites that do not support them? For example, why would you need to have portability for LaTeX like math equations on a site where nobody would use that feature anyway?
Point is that a “namespace qualified language-escape extension mechanism” does not look like it fits within a markdown document, i.e. it does not look like something the average reader would really understand the meaning of. Remember from the philosophy section of Gruber’s original markdown:
Readability, however, is emphasized above all else. A Markdown-formatted document should be publishable as-is, as plain text, without looking like it’s been marked up with tags or formatting instructions.
To be fair, LaTeX style equations aren’t really that easy to read in their source form either, something like the syntax proposed by @Kasper in Mathematics extension would be much more readable. But LaTeX style math equations are already established as a very needed feature to be able to embedded (and not handled directly by commonmark), and at least the reader sort of has an understanding of what it is or what it does (I wouldn’t call it that difficult to interpret, just not readable/pretty at a glance). And it should just be the math part of the LaTeX, not general LaTeX commands. (Similar to how mathjax doesn’t convert general LaTeX commands and instead promotes the use of native (html) elements for text-mode formatting.)
Besides why would there be a conflict with guys who want to use LaTeX notation with those who do not prefer it? If they are not discussing math/equations, why the need for the first group to use LaTeX notation/extensions at all? Shouldn’t they just be using the common syntax they can both understand and communicate to each other in?