There are relatively few individuals and organizations out there with products that are worth spending vendor money on, especially for something like a CSS library. Companies that do have this need are ready to spend BIG.
Tailwind charges a one-time fee in the hundreds of dollars range and pledges lifetime support.
When they say revenue is down 80%, it's because everyone already bought their library in its first few years of existence. And looking at their site there is nothing else to spend money on. So how are they planning to sustain their revenue?