Semantic-UI is a perfect example of the atrophy that I fear with combined CSS and JS frameworks. I'm pretty bullish on utility-CSS (I prefer tachyons since it's much simpler than tailwind). I don't think frameworks should necessarily combine the JS and the CSS. Ideally you would have a competitive market of people selling themes, using utility-css components so you're not limited by framework, and then you can just use your own javascript that you're going to have to use anyway (alpine, react, ember, angular, vue, etc)

I think if utility-css frameworks get popular someone will abstract the components from the frameworks and things like tailwind ui will become unnecessary (rather it will just be utility-css-ui and will work with any css-framework that implements the "interface").

Semantic UI is the perfect example of why you shouldn't go with something that's generic. Unfortunately the project just became too bloated for one person to handle. Your concern about sustainability is a good one and that's exactly why many of the frameworks' authors sell themes. However, themes usually require a lot of work and are difficult to implement using the actual css framework. tailwind ui is different because if you had the theme it's really easy to recreate it because it's utility first.