By that point though, you're dropping actual useful website features. What if I want a consistent style across my pages? What if I like restyling the whole website & I don't want to manually edit dozens of HTML files to do that? What if I think a header bar or a "recent posts" section would be nice?
I'm not saying it's bad to design a website in plain HTML, but it is critically limiting, and I wouldn't recommend it over a simple generator setup. If you're really opposed to using a fully-featured SSG, it'd be pretty simple to write a shell script that uses mustache(5) [1] to plug html articles into a template. That's almost as simple and clean as plain html, but it's a real step up when it comes to what it lets you do.