Comparing Svelte and React

Again, Redux is the purest expression of a state machine possible in a general-purpose programming language (as opposed to a specialized one like TLA+). It maps one-to-one with the usual textbook definition (technically textbooks don't have a rigorous definition for state machines, but rather finite state machines, but usually by "state machine" we just mean you take a normal FSM and just relax the finite bit by introducing some infinite component into the overall state such as an arbitrarily long list). The store is the state, the actions are transitions, and the reducer is the state transition table.

Redux's infelicities come precisely from the fact that it needs to interact with things that are not modeled as state machines! Namely, interacting with black box children and the outside world (mostly sending stuff to the outside world, Redux comes with stuff out of the box to deal with just one-way inputs from the world). And Redux doesn't come with ways out of the box to deal with those so you need to layer more stuff to make it all play nicely together. But you need at some point to interact with them to work with the JS ecosystem so you need that stuff.

If you had state machines all the way down you don't need any particular patterns other than the fundamental state machine description! Especially with Typescript's structural types, everything just works (you just pull out chunks of your store into little stores and pull chunks of your big reducer out into little reducers, it's literally just cut-and-pasting chunks of your code and giving it a new name and making new state machines all the way down).

The friction comes from interacting with React, dealing with the outside world, and figuring out various decisions of what to do about data structures that the JS stdlib doesn't decide for you out of the box (e.g. what immutable library should one use). Even the choice of mutable vs immutable is something that is decoupled from Redux proper (it turns out for incidental reasons it's easier for type systems to properly scope local effects with immutable data structures than mutable ones, but that's more an artifact of Typescript than Redux, see e.g. Rust's type system for a counterexample).

If all of that was just more state machines (and the stdlib issues were taken care of) then Redux would be absolutely lovely.

And that's not just a pie-in-the-sky hypothetical. E.g. Elm is basically what Redux would be if everything was state machines and works great in all the places Redux doesn't (Elm's infelicities in turn are of a different variety that stem from some limitations of its type system to properly express some of the structural types you'd like state machines to have, in particular a lack of structural union types that often requires duplicating the state transition table in certain ways as you break a larger state machine down into smaller state machines).

You skipped a huge step. There was angular before react. Which was horribly bloated and over complicated. People wanted something simpler. React, with it's facebook clout won in the end.

Are you sure "pure" is the right word? Last I checked redux very proudly incorporates a certain event-sourcing something idea in it. It's definitely not a "just state machine" library. More like in KFC when you order something, they always make sure that you get their sugar water as well.