This is an awesome idea and really resonates with me.
I did something similar for my site:
https://stratts.au/_editor/?preview=%2F&file=_site%2Fcontent...
You can edit and rebuild the site from within the browser, then download a .zip that contains everything. The editor can even edit itself.
Beyond self-modifying sites being just plain cool, there's a longevity aspect to it that I find very important as well.
There's also https://www.sparktype.org/ (a browser based CMS) which is also along similar lines.
This is lovely, but I think falls down the moment you want any generated content (table of contents being a typical first example).
What I was thinking, too. But the description of the design process by OP was interesting to read.
I was coming here to link it. Glad I reloaded the page before commenting to see your comment appear.