In that example, the requirements of the accounting system are also very poorly defined since they've evolved over time without thorough documentation. An email

In that example, the requirements of the accounting system are also very poorly defined since they've evolved over time without thorough documentation. An email system on the other hand has fairly rigid (but not perfect) requirements in the form of RFCs and other standards. I think the same applies to rewrites--systems with very well-known/understood requirements will be easier to rewrite than systems with soft requirements that have evolved a lot over time.