With webcomponents you are pretty close to the “metal”. If you know how to write good vanilla JavaScript, you can take most of that knowledge into webcomponents. You only need to learn the custom components lifecycle, and shadowDOM, which is knowledge about web-standards. With other frameworks you need to learn template syntaxing, how state propagates, how the compiler works, etc etc. Lot of that knowledge might be obsolete in 10 years.

Which isn’t to say it can’t be worth it. Learning multiple frameworks and libraries is also very helpful to skill up because you are learning about different concepts and implementations.