There's still a time and place for sending HTML and maybe a few tiny JS files down the wire, but for anything "application-like" it's just not feasible, unless your business is so big and performance matters so much that hundreds/thousands of engineering hours are a valid tradeoff to achieve it.
Regarding data loading and performance, there is a lot of exciting stuff in the near future - Remix loaders, Next.js RFC, React 'use' RFC, Quik, Astro, better bundling/build tools, etc.