> They didn't emerge from React - but it isn't possible to do React without them.

It is 100% possible to build large React apps without redux/flux. There are entire ecosystems built around, say Mobx, among many other libraries and tools which don't even follow the flux style patterns.

> The example on the website uses closures to handle component state

I'm not asking about component state; I'm asking how do multiple components access app-wide state. I can't use a singleton for that; how do I know when that singleton is updated? I guess I could use a proxy but I don't want to do that.

> and to let users pick their desired level of complexity.

React is the view layer. It doesn't enforce any type of data layer or state management layer.