Projects with a single gigantic CSS file also work and you can onboard people etc. That's not an argument.

they do work… for any value of work. I have been in the industry for a while, and I've never seen a project which doesn't slowly devolve into fights with the cascade over time - arbitrary abstraction layers, ad-hoc organization patterns, developer-specific conventions, inconsistent naming, and so on. All of this usually creeps in the more a project grows. At some point, someone adds postcss or scss or another preprocessor and starts splitting files, and then a whole new set of new, made-as-you-go conventions comes in: Files per component or per view? Or rather per architecture concern? Usually a mix of them. As files are split, the class naming gets even more inconsistent.

And at that point, onboarding a new developer always means wasted time on understanding this whole organically grown set of bespoke conventions, class names, patterns, hierarchies, and so on.

Tailwind does not really have this pattern of deterioration growing along both time and complexity, it stays consistent on both axes. So it's definitely an argument IMHO.