The big problem was not wanting to write even a small amount of is. After all, a simple web component in a page of code would have allowed client side includes.
I did think about that! However, I didn't like the idea of making the client side do work to make up for my own poorly-managed website. Forcing users to run code just to render some boring text seems like a waste.
If you set the bar to "0% JS at all costs" then you can't very well complain about how hard it was to maintain HTML.
Part of the standards, whether we like it or not, is JS. A sprinkling of JS (say, 60 lines?) to do client-side includes does not in any noticeable way increase the workload of the client, nor deteriorate the experience of the user.
After all, you provide a CSS file, right? The JS to do client-side includes using a custom element (so that `` works) is unlikely to surpass a modest CSS file in terms of size.