The extension language is most useful for the window manager, which should be running in the same address space as where the events are being generated and processed.
I think this was tried with GNOME Shell and turned out not to be such a good idea. It runs JavaScript in its Wayland compositor/window manager and that reportedly causes unreasonable problems with lag and unresponsiveness. The Purism people were advised to write their own compositor/shell instead for the GNOME/GTK-based Librem 5 phone.
Maybe that's a Wayland problem though, with the way it combines the windows manager with everything else into one process.
