Have you ever built an SPA with globally defined stylesheets? It's a huge pain that's near impossible to maintain. I still have nightmares from that time I tried to migrate a backbone application from Bootstrap 1 to 2.
It's not about CSS in JS, it's about having locally scoped CSS. Using JS to do it is just one approach.
Yes - more than one. The complexity is also a function of team size. I ran an SPA by myself for an entire organization and didn't have a single problem writing raw CSS.
Sure, everything is easy when it's just you. The value of having abstracted, localized CSS on a long-lived team with multiple developers cannot be understated.
It's not about CSS in JS, it's about having locally scoped CSS. Using JS to do it is just one approach.