Not only a loss but a key disabler. Having used to having the same customized window manager for decades it's impossible to change to Wayland until there's a fully equivalent interface for managing windows so that everything works as I want from mouse clicks to keyboard shortcuts. Maybe it could be an existing window manager adding support for River, or Wayback layer that reimplements an X11 desktop root on top of a minimal Wayland compositor, but none of the current Wayland compositors even scratch the surface of this.