LLMs could absolutely replace Tailwind, and if there was the need, aid creating something better.
I'll go a bit further.
I am 100% certain they will.
For LLMs to have replaced Tailwind - in part by using it themselves - this does not have mean that there _will_ be another library to reuse. In the context of LLMs, it becomes so "cheap" to customize the webpage that a library is no longer needed.
Tailwind in and of itself can be considered a "highly structured LLM" - if they so took it that far.