why is there no replacement for x11 forwarding over ssh??
There kind of is. The project you’re looking for is waypipe.
Knowing how these things tend to go, I predict you’ll try to use it for your use case and it just won’t work for whatever stupid reason. But I successfully used it to tunnel an app from my Debian machine at home to a Windows machine under WSL.
It’s also what makes FOSS niche.