One of the reasons that oil can more or less easily be validated is that there is a spec to the engine and the spec can be show to be invalidated when oil is not used.

Directly measured, the engine cylinder walls will grow wider and compression will be lost. Indirectly we can do an oil analysis and see the metals deposited.

One of the reasons we do not understand the cost of low internal quality code is that we do not know the "spec". Defect rates is one proxy measurement, but it's hard to know if it's just "bad programming" or "bad internal code quality" ...