Isn’t that what Phoenix (Elixir) is? All server side, small js lib for partial loads, each individual website user gets their own thread on the backend with its own state and everything is tied together with websockets.
Basically you write only backend code, with all the tools available there, and a thin library makes sure to stich the user input to your backend functions and output to the front end code.
Websockets+thin JS are best for real time stuff more than standard CRUD forms. It will fill in for a ton of high-interactivity usecases where people often reach for React/Vue (then end up pushing absolutely everything needlessly into JS). While keeping most important logic on the server with far less duplication.
For simple forms personally I find the server-by-default solution of https://turbo.hotwired.dev/ to be far better where the server just sends HTML over the wire and a JS library morph-replaces a subset of the DOM, instead of doing full page reloads (ie, clicking edit to in-place change a small form, instead of redirecting to one big form).
Idk about Phoenix, but having tried Blazor, the DX is really nice. It's just a terrible technical solution, and network latency / spotty wifi makes the page feel laggy. Not to mention it eats up server resources to do what could be done on the client instead with way fewer moving parts. Really the only advantage is you don't have to write JS.
It's basically what Phoenix LiveView specifically is. That's only one way to do it, and Phoenix is completely capable of traditional server rendering and SPA style development as well.
LiveView does provide the tools to simulate latency and move some interactions to be purely client side, but it's the developers' responsibility to take advantage of those and we know how that usually goes...
It's extremely nice! Coming from the React and Next.js world there is very little that I miss. I prefer to obsess over tests, business logic, scale and maintainability, but the price I pay is that I am no longer able to obsess over frontend micro-interactions.
Not the right platform for every product obviously, but I am starting to believe it is a very good choice for most.
Basically you write only backend code, with all the tools available there, and a thin library makes sure to stich the user input to your backend functions and output to the front end code.
Honestly it is kinda nice.