Your devops team assures software quality?

But well, no QA effort is complete enough to really assure the quality of a large system. Thus, the GP holds true.

Nonethless we are building and operating complex systems.

Testing something 100% has diminishing returns.

Therefore -> you will never be able to prevent all issues. As you are not aware how many changes are happening, you don't have a value which could indicate the healthiness of it.

I personally think that when i'm getting older and better in my job, i still make errors, the amount and severity is going down though.

I'm sure they have both but complex shit breaks when you change it often.