We should work out a sensible strategy to live in a world where the phrase "wiki markup" becomes increasingly synonymous with “Markdown”
IMO that world is one in which the MediaWiki (and wiki) technical community has failed (that includes me), as the world overtook it with a less useful format, and the generic terms 'wiki' and 'markup' have lost their meaning, and the Wikimedia content is stored in a format that is no longer a format that MediaWiki believes and invests in.
But I am not seeing that world, yet. Where is it coming from?
Which wikis support Markdown? I only see w:GrokOla (software) (proprietary), w:Gitit (software) and w:PmWiki which mention markdown on w:Comparison of wiki software, and 'Hazel' on w:List of wiki software. If there are more wiki software, we should document them on Wikipedia and/or here.
The ones most of us developers will be aware of are Github, BitBucket and Gitlab
BitBucket only supports Markdown as far as I can see, but Gitlab also has w:RDoc and w:AsciiDoc (both of which are far better than Markdown, in features and in standardisation/specification).
Github wiki supports multiple formats, including markdown, of which mediawiki syntax is one of them, using WikiCloth listed on Alternative_parsers.
How can we avoid that world?
Invest heavily into Markup spec, and into alternative/reference implementations, including providing a saner version of the syntax that cater for the needs of other organisations that consider the security and processing overhead of wikitext to be problems compared to markdown. It should be the default for new MediaWiki installs, and older installs should have a nice tool that converts the crazy unspecified wikitext into 'wikitext-simplified' where possible.
Work with other wiki vendors that might want to make 'markdown' syntax an implicit choice, not clearly identified as markdown syntax, stressing that causes confusion for users.
And work with other wiki vendors to increase compatibility between their syntax and ours, so that 'wikitext' and 'wikitext-simplified' are viable long term formats.