Mobx solved all of the state problems properly a long time ago.
I'll agree that the Redux DevTools "skip action" and "jump back to action" features are not all that commonly used in practice. I _maintain_ Redux, and I don't even use them that often.
On the other hand, the ability to see a written list of all dispatched action type names is valuable by itself. So is the ability to click one of the listed actions and see the action contents, state diff, and final state. _That_ is very powerful.
Beyond that... I now work at a company called Replay ( https://replay.io ), and we're building a true "time traveling debugger" for JS. Our app is meant to help simplify debugging scenarios by making it easy to record, reproduce and investigate your code.
The basic idea of Replay: Use our special browser to make a recording of your app, load the recording in our debugger, and you can pause at any point in the recording. In fact, you can add print statements to any line of code, and it will show you what it would have printed every time that line of code ran!
From there, you can jump to any of those print statement hits, and do typical step debugging and inspection of variables. So, it's the best of both worlds - you can use print statements and step debugging, together, at any point in time in the recording.
Additionally, because Replay records the browser's OS calls, it captures _everything_ that happens in the page. That means you can debug _any_ website or JS app, no matter what framework it uses - React, Vue, Angular, Svelte, jQuery, or vanilla JS.
I actually recently implemented a POC version of support for the Redux DevTools in our Replay debugging app, so that if you do record a Redux app (or Jotai, or Zustand, or NgRx), you can use that same Redux DevTools UI to see the action history.
So, yes, time travel debugging _is_ an amazingly powerful concept. It's just ironic that that particular aspect of Redux didn't end up getting used that much... but the Redux DevTools themselves are still valuable, and Replay is actually a far superior "time travel debugger" overall.
What I meant though, was that people get hung up on the some ideal of how Redux is and will work for them while the reality is often quite different.
What problem does MobX not already solve?
I’d say the two biggest hazards with the reactive/declarative style are cyclic dependencies in the data model and remembering history.
Tools like MobX let you write quite elegant code in the right circumstances but they are less helpful if, for example, you have a complicated set of constraints and the effects of changing one value in your state can propagate in different directions depending on what first changed to start the cascade.
This style also tends to emphasise observing the current state of the system, so if you need a history as well (for undo, syncing with other user actions via a server, etc.) then you probably have to write a whole extra layer on top, which is more work with this kind of architecture than for example if you are using a persistent data structure and reducer-style updates (as popularised by Redux within the React ecosystem).
So there are some solutions available if normal mobx is not enough.
If you like that model, Svelte stores are very close to that as they are really reactive primitives. Honestly I think it was a terrible idea to call them stores.
SolidJS is also built on this idea of using reactive primitives. In this regard it's even better than Svelte as you get true fine grained reactivity.