It’s a poor metric because without the file being present, Github tells you to create a file called README.md. Not a file called README.md or README.markdown.
If you don’t see that as a black mark big enough to make the metric useless, I’m honestly not sure how why we’d continue this conversation.
In standardizing a thing explicitly not to be standardized, a new thing is created. That appears to be recognized, as a new name was picked. A new extension should be picked as well.
It does not prevent Markdown from being processed by Commonmark. It prevents Commonmark from being processed by Markdown.
The half in/half our nature of this project will be its undoing. I’ve already lost most of my interest in the project as a whole over this thread.
“.md is better because it is shorter I don’t want file names to take my whole screen in width”
We’re not living in the world of 8.3 file names for a long time now. The same argument could be applied to .torrent as well, which is only one character shorter than .markdown. Short extensions can be nice but .md is already ambiguous even without Markdown.
For what it’s worth, it seems as of revision 6292, Vim recognizes all .md files as Markdown (not counting possible ftdetect magic). The first release that includes this is Vim 7.4.480 (based on revision 6293).
$ hg blame runtime/filetype.vim | grep -i markdown
[...]
6292: au BufNewFile,BufRead *.markdown,*.mdown,*.mkd,*.mkdn,*.mdwn,*.md setf markdown
6292: " Modula 2 (.md removed in favor of Markdown)
[...]