[kstenerud]

When the cost of failure is low.

Rewriting involves introducing new bugs and incompatibilities with existing systems. It means delays, data corruptions, and failed milestones. It means disrupted routines and failed expectations. It means pissing off a LOT of people, and so the biggest risk is political. Chances are pretty high the project will simply be canceled once people see it in action and understand how difficult their lives will be during the "transition", which could span years.

Don't rewrite a system people actually use. It'll be your head on the chopping block.