See “reasons for wanting this” in my original post above. In short: performance, SEO and accessibility.
Spinning up a full (headless) browser on the server is needlessly resource-intensive, and not especially fast unless you’re maintaining a fleet of “hot” browsers waiting to render things.
Yes, but it’s a case of “why not both?” Rendering with Elm is faster than React (say), and doing the initial render on the server is faster than doing it on the client. Combine the two for maximum speed.
Again, these are all beneficial performance measures. One does not preclude the usefulness of another.
That said, there would be value in measuring their relative benefits in real-world scenarios to see which should be the priority if people need to choose one to focus efforts on.