Why a previewer rather than an editor that updates as you write?
Do you have a specific use case?
It seems to me that markdown is for writing with the ultimate output supposedly being html. Having a viewer of the markdown doesn't seem to add anything.
Whereas making it an editor makes it more of a rich text editor.
I'm not particularly saying youre wrong, more posing a philosophical question.
There are both "open in editor" and "watch" modes.
The idea is not to replace an editor, but to complement it: - "open in editor" lets you edit the file with your preferred editor - "watch" automatically refreshes the preview when the file changes
So you can keep your usual workflow while having a fast, structured preview directly in the terminal.