The goal isn’t to prevent escalation to root, but to prevent escalation out of a sandbox. If you sandbox an application today, while it can’t access the filesy

The goal isn’t to prevent escalation to root, but to prevent escalation out of a sandbox. If you sandbox an application today, while it can’t access the filesystem at all, and nothing under your user account, as long as it can show you a window under X11, it can log, manipulate, and inject any input events or window contents of any other windows as well. That’s the one single big hole in todays sandboxes under linux, and there’s only two options to solve it: put a whole separate X server into the sandbox of every single application with X-on-X, or replace X. Which is one of the things wayland fixes.