People don’t or even can’t remember how was front end development before React/Flux/Redux. You could easily had problems with state management even with less than 1000 LOC simple pages. Of course, you could mitigate it, but it wasn’t trivial at all.
Backbone employed a two-way data binding flow. You're responsible for updating the models (ie. state) (way #1) when the user triggers events, AND you are responsible for updating the DOM whenever the models (ie. state) changes (way #2).
In React, they used a revolutionary new paradigm (flux), making it so you only worry about one direction (updating the models/state in response to events); you never render anything (React renders everything for you in response to state changes)!
If you've tried developing a non-trivial site with both, it quickly becomes apparent how much that one difference completely simplifies a huge aspect of development.
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Not sure if React isn't being presented as such anymore, but that's still the problem I see React solving, not more, not less.
As you say many people do not understand how important, vital and bizarrely _non-obvious_ uni directional data flow was.