I think React’s biggest win is that it appeals to the programmers, not just the web developers. For the first time everything you need to make a website or webapp — html, css, etc – is just data. You have direct control, direct access, you can do what you want. There’s a beauty and simplicity in that. React is to web frontend what Lisp is to systems programming.
The downside to that is that I've seen a lot of old school frontenders who 5-7 years ago were able to just dus html/css and sass haven't been able to keep up, it just got too complex. In that regard the old situation had better seperation of concerns.
I recently wanted to throw up some minimal stuff on S3 without building, just a HTML file, and was surprised at how small the difference was, for simple enough pages - almost 0.