You're not learning anything valuable by learning Tailwind. Learn CSS, that will stick with you for the next 20 years.You can't use Tailwind without knowing (quite a lot of) CSS. There's nothing in Tailwind that makes CSS easier or that obfuscates the technical understanding you need to make things with CSS. If you want to make a horizontal flexbox layout knowing "display: flex; flex-direction: row; flex-wrap: wrap;" or "flex flex-row flex-wrap" are functionally identical. You need to know what either the CSS or the Tailwind classes are doing to use them, and there's a one-to-one mapping between them. If you learn one then you know both. It's literally just a shorthand notation, or in a few places the equivalent of a macro.
I don't imagine Tailwind is going to last another 20 years, but I do believe that people will use 'utility first' CSS libraries like it from now on. No one is going back to writing plain CSS unless they have no choice. CSS is becoming increasingly capable, and with that increasing complicated. People choose Tailwind because it affords you all the power of CSS with a somewhat less arcane syntax. Why would anyone stop using that?
I do have a caveat though: I actually think Tailwind is the CSS equivalent of jQuery. In 5 years or so a browser is going to add a Tailwind style notation to its engine, and people will be able to use it without needing an external library. In that sense, yes, Tailwind will die, because browser developers will see the benefit is so significant that they'll adopt it as well.