What AI replaces isn't Tailwind (AI will reach for Tailwind pretty readily), but the paid ecosystem that was Tailwind's business model. The value of having a pre-built component goes down when building that component is a 10 minute job of an agent.
Their funnel depended on SEO, and LLMs took SEO out of the equation. What you said about the value of pre-built components might also be true but can't be determined from a situation where consumers suddenly all stopped learning that the pre-built components exist. Occam's Razor says it's the funnel.