This has indeed been brought up a few times…
- Plain Text support and syntax highlighting
- Customizable automatic Highlighting
- rich text driven “code hiliting” for markdown in editor?
- plain text
- Future wishes for MMD integration
- …
And that was just searching the wishlist tag for “syntax highlighting”.
As you will find, the main problem with this idea would be: are you willing to give up images, styles, annotations, highlighters, footnotes, cross-references both functional and personal to research, &c? Perhaps, but I don’t think most would, and it would be supremely awkward to have a majority of the menu system and a whole toolbar disabled in such a hypothetical mode.