React was not a bad idea. SPA's tend to be a bad idea. React is just a tool to make SPA's easier to write.
Svelte is ok. It could have been great but the api for their version of observables is a disaster (which I hope they eventually fix). Sveltekit is half baked and convoluted and I strongly advise not touching it.
VDOM is also a good idea that simplifies the mental model tremendously. Of course these days we can do better than a VDOM. Svelte in fact doesn't use a VDOM. You can say that VDOM is a terrible idea in comparison with Svelte, but that's just anachronistic.