Specifically in the context of a web application, how good or bad would be if we store everything (yes, everything) in one giant object? No private state anywhere.

Stuffs like the Elm architecture and the tons of state management thingies that are based on that idea make me hope. I like that they allow you to write a "view" as a function that only depends on its input. But people in the javascript world just treat it as a convenient way of passing state between components that are not "close" to each other.

Another thing I can't stop thinking about is that a firebase database is a json object. Putting a lot of things in objects is not that crazy.