Sandboxes have often been represented as a security feature, but it seems there are always ways out, e.g. there are always jailbreaks available for iOS. Sandboxes still make sense for restricting non-malware apps from accessing private info. For security, its more important to avoid targeted exploits, e.g. reducing identifiability, to force the use 0days into broader targets, which is then more likely to burn that 0day. Users of 0day exploits are rarely willing to burn one to target one person