It still matters if the error occurred when the life expectancy was 7 years or 7 months, so ideally you'd want to count that.
There should not be any errors, period.
Yes, you can find mitigating factors, whatever. But that the end of the day we want to reduce all errors.
At the end of the day we have to accept that the errors will be there, no matter what you or doctors do, period.
And as in all other industries where errors have a high impact, you have to design your process / systems / procedures so that you get an acceptable result even if people make mistakes, and it takes a combination of multiple errors (or an error combined also with negligence) to kill someone.