Can you explain what it means to pollute the CSS namespace and why it matters?

Are you sure you are correct or are you just guessing?

Just stop being so confrontational. Styled-components enable you to use SASS, not having have to invent any classNames or worry about them conflicting, enables you to easily extend those styles and gives you the ability to mix code with your styles in kinda effortless way. Okay granted that mixing code is not as clean as I'd like it to be but still I haven't found a better option

You name a class ".button" and it will conflict with completely unrelated component's style tags which also have a class named ".button". You don't have this problem with CSS-in-JS. You isolate the styles completely.