1) Flying is incredibly safe
2) Tired, fatigued pilots crash planes
3) Airlines are low-margin businesses
4) There is no economic incentive to have pilots fly less and sleep more
Since there will always be a tension in allowing pilots to sleep more and work less (thus costing airlines more money) we have to conclude that pilots are going to fly without sufficient sleep.
To make the system more resilient to failure, we should take the ad-hoc napping that pilots do now (which is undeniably adding some safety via fatigue reduction) and add a blame-free procedure around it to ensure safety (e.g. when one pilot goes to the bathroom the other one puts on their oxygen mask in case of explosive decompression; something similar should be done for the awake pilot.)