So you want every markdown parser to support building diagrams as well? That seems much more difficult than offloading it to an open source library. Why waste effort that already has gone into building an easy to read diagraming language?
The real criticism of the original comment is that markdown has never really been "portable"; yes there's now a hard dependency (for some definition of "hard") on mermaidjs for GitHub-flavored-markdown, but that actually brings it in line with other implementations. So has portability been obviated or just slightly changed?