The payload probably won't increase as much as we'd think thanks to gzip, and you can apply the styles via server-side rendering rather than waiting for the JS to load.

We also need to consider the time from request to a ready page: if you're on an incredibly slow connection, then having the styles immediately rather than waiting on a second round-trip is a huge win. I agree that there are pros and cons, but some of the pros are actually in performance; it's not immediately clear which technique is more performant overall in what contexts.