Sandboxing does not mean no interaction. It means supervised interaction between apps where the user is in control of what to allow and what not. I'm not sure how well Flatpack does it (never used it) or Snap does it (used quite a bit but never tested how well it sandboxes) but there are other platforms that handle sandboxing pretty well and then provide APIs on top to allow interaction with the system and other apps (iOS, Android, Meego for example).