Here's what killed web components: lack of native databinding on the web. That's the reason the standard is useless without JS. Any modular, dynamic, modern UI requires databinding, which means it's going to bring in a framework anyway, which means that self-contained widgets are all going to bring in their own frameworks, which means that instead of one large framework on the page you have five or six, all of them stitched together through rickety HTML-attribute APIs and custom value-parsing strategies because HTML attributes are just strings.
I fail to grasp why databinding hasn't made it into a web standard yet. The web has a long tradition of feeling out features in the JS world before adopting the successful ones into the platform. jQuery turned into querySelector() and fetch(), CommonJS modules blazed the trail that led to ES modules, etc. And this next paradigm is more than ready to get standardized. Not only would it make the dream of web components possible, it would eliminate the need for a whole lot of the JavaScript out there and even make UIs faster, since reactivity logic would be implemented natively.
It's such an obvious, ubiquitous improvement to the web that I can only assume there's some fundamental implementation roadblock I'm missing.