This might be what you wish was the standard, but it really isn't. If anything is “the de facto standard” it’s WordPress, which runs about 30 percent of the web. A tiny proportion of primarily tech-related businesses and a lot of developers use markdown and git.

Most people who write and publish on the web, including professional writers, do not know Markdown and have never heard of git.

As someone who works in marketing and publishing, I can tell you that it is enough of a struggle to stop writers using Word—plain text and git are unthinkable for most of them.

That's absolutely laughable. I'd give XML much better chances to be a future-proof markup, and I'm not even a fan of XML.

Especially considering the dozens of MD variations (Vanilla? Extra? GitHub? CommonMark? pandoc?) preclude any future-proof-ness. And all the ad-hoc extensions you do in your application when you want to support (for example) figure tags with alt text, title and caption (not even really possible in pandoc) make things even worse.