The vast majority of modern websites could easily deliver their existing functionality (except with better interactivity) with server-side rendering and just enough plain-vanilla javascript to handle ajax calls.
If you don't believe this, I suggest you carefully reexamine your assumptions. A number of them are almost certainly false.
Don't even need Ajax anymore. You can render on the fly, and deliver new HTML over websockets. The gap between SPA and classic server-side rendering is getting smaller fast.
JS is still required. Sending plain HTML is not the best path forward.
The idea in this case is that you'd have a bit of JS that would take care of updating the DOM, binding event handlers (which is specified and taken care of server-side), and so on.
How Phoenix.LiveView would do this exactly is still being worked on, but [Drab](https://github.com/grych/drab) will probably offer some inspiration.