Not the OP but I'm happy to defend WYSIWIG editors. Things like font size and bold and italics exist for a reason. Markdown tables are extremely clunky. Inline images (or pretty equations) are important for many types of writing. Bullet lists in markdown more than two levels deep get interpreted as code my most syntax highlighters.
Don't get me wrong, I love plain text and I do most of my writing in markdown, but WYSIWIG editors have their place. As do things like spreadsheets (for represent lots of info) and LaTeX (for docs).
The general problem, of course, with WYSIWIG is distribution: what YOU see might not be what someone you send the document to sees. Do the people you're sharing the document with have the same application, the same version of that application, the same fonts installed? You can export to a presentation format like PDF (or HTML), but then they can't edit it as you did, and it's also likely that the translation will change "what you see".
Markdown is a least common denominator for sure, but that limitation also gives it a useful universality, which is what proponents value in this case.
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> Better tools for composing documents already exist.
Such as? Honest question, and a hope you don't mean MS Word or Google Doc.