Next.js without a node backend, what do you lose (SSR, etc.) by choosing to use another backend-only or monolithic framework?

I was looking at Rails (API mode + partial asset pipeline) + Next.js for a project to start, and might entertain elixir or rust when scalability of slow areas became a problem. Sometimes I think there will be eventually a system of LLMs that will figure out how to manage and configure operating systems, write mobile native and web apps, and administer databases. And then they'll eventually rewrite programming languages and design silicon to be better for themselves, so that they will only need a part time human to clean and maintain the robot security guard.