Basic networking. What's your redundancy? HSRP? What happens when someone spoofs your VIP/virtual MAC? Everything is down. I've seen large offices taken down becuase they used 192.168.1.1 as an important device, and someone plugged in a home router under their desk as an AP, causing a conflict that took down a "redundant" network.
Someone can always take it down. So, go for 802.1x on every port to combat that. Now, if you radius server has an issue, nobody can work. Brilliant. Redundancy and security reduce stability. Go back to networking 101. Even redundant SUPs in a chassis-based system have a single linked management. One wrong command in one of the SUPs and you can take down everything. Redundancy rarely survives user error, and makes it harder to bring it back up.
Um, you do realize that there are networking technologies to protect the network from practically every scenario that you mentioned? Even your port security example falls flat on it's face because you would have redundant radius servers, etc.
And no, redundancy doesn't make things harder as long as it's implemented properly (i.e. you have a well documented primary path that's always used unless there is a problem in which case the network switches to a well defined backup path). As for redundant Sups, most large companies that need up-time have redundant hardware/chassis rather than relying on line cards in a single chassis. Removes the human error and hardware failure. Because you are right that the biggest problem is human error.
The key to any network implementation, same as any other IT service, is that you have experienced network engineers to architect the network, well defined standards, and good support engineers to run it and follow the standards.