Not every legacy system deserves a migration. Some deserve a façade — a carefully engineered layer that lets the rest of the business move without forcing a rewrite.
We use a four-question framework to decide. The questions aren't about technology — they're about the business clock: regulator timelines, vendor support windows, the cost of an outage, and the cost of a rewrite that ships late.
This essay is the framework, with three worked examples from banking, healthcare, and industrial operations.