You're thinking of open source projects that only need a few hours of work per week. Anything more than a few hours a week either requires someone to give up their full time job to work on it (switch to part time jobs/consultation is an option), or having multiple contributors which still require significant effort to coordinate at the end of the day.

Let's say Tailwind CSS gives up and stops the project, do you think there'll be someone else picking it up, knowing how Tailwind failed in the first place? LLMs don't create new things, they remix what are already available. It's delusional to think that you should use LLMs to create a whole UI library just for your application and spend enormous effort not only maintaining it but also train new team members to use it down the road.

Open source is charity, it's unreasonable, even entitled, to demand someone work on it full time without pay.