Imagine a programming language that only had global variables. You have to carefully name your variables using complicated conventions in order to avoid any conflicts or unexpected behavior. Even your functions don't have local variables; there is no lexical scoping. Any function can overwrite or mutate what any other function is doing inside. There is no real encapsulation. After you've done this perfectly, your app…
I don't entirely understand why some people find CSS so difficult to work with. I agree that it has its own challenges, but solutions generally aren't very complex. Global selectors ("variables") collisions? Use namespacing. Specificity is convoluted? Simplify by using only non-nested classes. Does the annoyance primarily center around the fact that styles cascade to child elements? Hard to tell from the article.
Let's say you wanted to pull in a button from Bootstrap for one part of your page, a button from Material UI elsewhere, a button from Semantic UI elsewhere, etc.
These things are made to be reusable right?
What are the chances you can do that without issues? Do you think any of their style rules/selectors would clobber each other? What about the base/assumed CSS that they all most likely require? What about components more complicated than a button? Without actually trying it, how much confidence do you have in your guess, vs. your assumptions about importing other things (like functions from npm modules)?
Good software is reusable without worry.