One argument that I've got from biology is: The simplest possible system (code, living organism) is fragile. Any change, however minor, is likely to break it. A nice chubby complex system, on the other hand, is likely to be more resilient. You may change it a bit and it's still likely to work somehow.
You should take a look at tardigrades then (aka water bears), which are approx. 1mm in length, obviously less complex than most living creatures, yet are some of the most resilient animals on earth; highly resistant to extreme temperatures, pressure, radiation, etc.
One way to look at resilience: a relationship between the system and its environment. So when you say resilient, you need to specify, resilient to what? I think a resilient system, is neither too complex, nor too simple, but rather, adapted to its environment.
On a related(?) path see https://en.wikipedia.org/wiki/Exaptation