for what it’s worth, personally I equally dislike both index.md and _index.md, and even giving a full path in the frontmatter like "url: writings/index.html". Personally, I would really prefer to be able to say in the frontmatter something like:
isindex: "true"
to tell hugo “make of this file the /index.html of the folder it is in, period”. That way, I could give all my files whatever unique filenames I wanted, and without having to rewrite the url: keywords if I decide to move folders in other places.