I'd like to point out that "CSS-in-JS" is an umbrella term, and that there are lots of takes on actual implementations of this. It's possible to like one approach and not another.
My guess is we'll end up with a split down the middle as a best practice someday.
When we write styles, we will always make a choice. Is this a global style? Am I, on purpose, leaking this style across the entire site? Or, am I writing CSS that is specific to this component? CSS will be split in half between these two. Component-specific styles will be scoped and bundled with the component and used as needed.