Maybe you can upgrade digital systems until they are 'fully fortified' in theory, but humans have an upper limit what they can understand / perceive / critically analyse (not to even speak about engineered high pressure situations) that can't be upgraded.

There is an answer to that - simplify and abstract. Lots of human software is unnecessarily complex, often caused by backwards compatibility and general human creativity.

Take sendmail vs postfix as an example of this process.