Okay, so go before that the jQuery (should win the Nobel Peace Prize) used with vanilla JS building absolutely bonkers custom scripts all over the place.
React was a paradigm shift towards more complex frontend apps, but there was still complexity. It replaced a bunch of .erb or mustache or whatever templating that then tried to be interactive with JS layered on.
What React replaced was not less complex overall, though technically I guess it moved more of the functionality to the frontend.