This is already here and in use. See Next.js using React.js for example. Server-side rendering out of the box and small next needed pieces are loaded continuously when they are needed.
My experience is that most of the time it’s better to make your public facing site fully SSR and then make the actual app fully client side rendered. You usually don’t need that much interactivity for your public site and your app users don’t mind a few seconds startup time. You also usually don’t need SEO for your app because it’s behind authentication anyway.
Exceptions exist of course.
Those are basically the same drawbacks as a decade ago.
I guess my question to the folks here saying hybrid is the way to go nowadays is: Have these problems been solved in your view?