> the designers and implementers come up with data models and workflows that feel like they cover every possible case and they deliver a system that is a perfectly rigid implementation of these flows because they seem complete. Inevitably, these systems meet the messy real world and users find themselves unable to do what they need to do for the occasinal really weird case and get hopelessly stuck because they run up against stubborn rule checks.

No. Those systems have nothing to do with the real world. They are just a project manager's idea about "the real world". Have you used any Microsoft product lately ? Every day it tells you: "Hey , i have a new feature".

Just stop.