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.
It is described in the Tailwind docs here - https://tailwindcss.com/docs/utility-first/
Tailwind-UI looks very interesting and I would have no problem paying for it, rather than getting a "free" version, if only for the long-term sustainability. Community projects often go through endless rewrites, and suffer from feature creep and lack of direction.
In a business setting (I run a self-funded SaaS business) I am much more interested in a framework that fixes bugs and is stable.
The only thing that worries me is that the pricing is one-time rather than subscription. How can the authors expect long-term sustainability with a one-time payment? Maintaining a CSS library is a continuous effort, not a one-time thing.