> 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.

Yes. Every feature is also a vulnerability. The most secure features are the ones that don't exist.