This implies that "hydration" exists exactly how it exists today in SSR. That's not the only way to hydrate a frontend client. You could, for instance, as part

This implies that "hydration" exists exactly how it exists today in SSR. That's not the only way to hydrate a frontend client. You could, for instance, as part of a server payload send back a JavaScript object full of state and the frontend will read from it and render accordingly. But that would require not using a framework and building it yourself, which I think developers nowadays aren't keen on doing.