It is, but my argument is that better security is not needed! There is only one user. All apps are his/hers.

Linux security inside a user's session is bad in general. It all started in the '90s from the principle that all installed software is FOSS, sources inspected, and compiled distro packages trusted.

  - Permissions are per-user, not per-app. If I ever need permissions to something, every app I run gets the same permissions.
  - No isolation for personal files. Any app running as me can read all of them. Bookmarks, firefox passwords, everything.
  - No per-app firewall mechanisms. Any app can call home and send user's files, including those passwords.
  - System events are broadcasted via dbus to any app wanting to listen to them.

So why make the GUI secure if everything else isn't? If anyone wants to have better security (or privacy) in Linux, would have to start with the basic permissions model.