Letting any GUI application capture all input and take full control of the desktop completely defeats the point of sandboxing and X11 does exactly that.
Sandboxing defeats the point of said applications. If you want your computer to have no functionality, check out Figma. A clickable prototype sounds like precisely the security the world needs right now.
The only thing keeping those away from Linux was its market share. With npm malware on the rise, this is no longer enough of a protection.