Frontend/backend is a speed of light issue. As long as the speed of light remains constant (and I've not heard any plans to change it) there will be a need to a have some code on the client and some code on the server.
The latency of a communication travelling at the speed of light from say Japan to New York, is noticeable to humans. If you've ever tried to SSH into a machine on another continent, you know the feeling.
Even if the server is nearby, there's still many things that can add latency, most of which are not controllable (e.g. wifi interference, bad routers, bad ISPs, train tunnels).
So in general you cannot move all functions to the server because it will be terrible for UX. e.g. Google Stadia.