TLDR; don't change the writing experience, just add markdown syntax support.
I'm a big fan of markdown, but as with most things I've used for a long time, my tastes are very particular. The particular editor in 200wad doesn't match those tastes.
First, I love the original editor. I'm big on writing straight onto the page. I refer to a lot of content creation web apps as "white boxes" but my favorite kind of white box is one you can't even see. The new markdown editor is definitely a white box.
I don't love the formatting tools of the original editor, but I don't need them very often.
Second, Grammarly doesn't pick up the new editor. I've grown really attached to Grammarly, especially within 200wad. It mostly gripes about commas but its spellchecker is great and I love seeing my stats every week.
Third, are those particulars I notice in a markdown editor. Cmd+K in the 200wad editor automatically puts the HTTP prefix into the link section, which is entirely unnecessary since I'm almost always pasting a full link in there.
Who does markdown really well? Well, text editors, for one. But it's unfair to compare a native text editor with a web text editor. Github's markdown support is pretty stellar.