Defense in depth through sanboxing is there to make sure that even an attacker that get some amount of undue privilege will not be able to do harm. At this poin

Defense in depth through sanboxing is there to make sure that even an attacker that get some amount of undue privilege will not be able to do harm. At this point we've learned that all existing systems will have vulnerabilities of some sort, so defense in depth is the only credible security posture.