That complexity is excessive like a human who would taste everything in the grocery before buying everyday his bread ! We are using to much software, to much energy, aso that we don't need !
Of course using tested modules and defensive layers is not far to be mandatory to chase the zero-bug level.
"99.9"... Once again that number is just terrifying . We had that discussion with PJ2 already !
Zero is really very lower than .1 !!! The limit to zero we have to search is given by the time of Planck ! Mostly people don't know which "zero" they need and decide to reach ! Having one chance/hundred to die going on the moon the first time is acceptable, but 1/100 or 1/1000 in FADEC failure in a airliner engine is not acceptable. You cannot say you did debug your software if you don't know exactly the risk and price of the failure of that software .