Writing markdown is decidedly not WYSIWIG. Storing documents with a WYSIWIG editor in a markdown-like format isn't viable either - look how frustrated word processor users get if changing one element repositions another ("all my pictures move"). Users want things in a precise format.
Nobody is proposing that people will be writing markdown by hand in text editors. Just that a lot of the complexity of rendering stuff on the screen precisely as it would be printed on, say, A4 paper can perhaps soon be left behind. People print a lot less these days, and thus WYSIWYG (in the original 70s, 80s and early 90s sense of the term https://en.wikipedia.org/wiki/WYSIWYG) may be less important.
