It is slow and painful and requires you to pay lots and lots of careful attention to the behavior of the existing enterprise system that you may very well hate.

It is slow and painful and requires you to pay lots and lots of careful attention to the behavior of the existing enterprise system that you may very well hate. Greenfield development sings a siren song. You get to scribble on a gloriously empty page in your imagination, free of such mundane concerns as cash flow, near-term customer demands, day-to-day stability, vitally important edge cases, and hard-won but crufty bug fixes.