React’s changes over time have always been broadly compatible, and the same under the hood, just presented and manipulated a different way at the surface. Migrating to anything like Svelte would be a radical and extremely incompatible change on multiple fronts. It’s never, ever going to happen; the closest you’ll get will be another layer on top of React that embeds something like Svelte—such as https://github.com/Rich-Harris/react-svelte.

Svelte’s approach requires detailed knowledge of the structure of state, and requires compilation: components’ <script> blocks are not written in JavaScript, but rather a language with the same general syntax but different semantics, and some places where JavaScript is too flexible to be tractable get replaced with special template syntax (like {#each} instead of for-loops or Array.prototype.map). Svelte cannot be implemented as a JavaScript library (I disqualify eval()). Svelte is also deliberately severely limited in what it can express in various places, whereas React gives you the full power of JavaScript (for better and for worse).

You could perhaps implement an optimising compiler for a small subset of React components that avoid problematic patterns and are written in TypeScript with proper specifications of the types of state and props; but if you considered it unacceptable for this compiler to change the component’s semantics, I think you’d be surprised at how little serious React code in the wild could actually be supported. Even simple loops might be out of reach. The Svelte approach can’t be a progressive enhancement, it’s an all-or-nothing (at the component level).