It's great to see people moving in this direction, but I'm disappointed that everybody has decided to reimplement basically the same thing independently.

Mobx was ahead of the game here (though, granted, it too draws on Knockout.js). You can use Mobx to declaratively describe reactive state, much like this. But it isn't integrated into any framework - you can use it in vanilla JavaScript with no other runtime or compilation required. You define models with Mobx, then on top of that there's mobx-react, which ties your model into React's update APIs (essentially making render() automatically observe values, and making relevant changes trigger a re-render), or there's mobx-vue, or mobx-svelte, or mobx-preact, etc etc. Decoupling this is super helpful - for example I'm using mobx to model raw state, computed state & reactions to changes in server side Node.js, no problem.

Meanwhile, recreating the same reactive concepts independently in each library instead makes all the resulting code incompatible, so even your pure JS state models are coupled to your UI framework - e.g. createCounter in this post has to import & use $state from Svelte. That makes it far harder to change frameworks in future, hard to share code between apps using different frameworks (plausibly even different versions of the same framework), etc etc.

I'd love to see a native common standard for this, similar to how promises were eventually standarized and suddenly everything had a compatible model for handling async future results (I could swear I've seen initial discussion on exactly that already, but I can't find it anywhere now sadly).