Its funny, when you tack the “legacy” adjective to a system it suddenly is something that must be replaced. However, like any other asset whether or not a legacy system should be replaced is determinable by a cost-benefit analysis. The following matrix is a good way to decide whether to replace a legacy system:
|High Business Benefit||Low Business Value|
If a legacy system is a high quality system then there is no need to replace it. This follows the old cliché: “don’t fix what isn’t broken”. If a system has low quality and low business benefit (measured by how many people use it and how vital it is to their job) then its better just to scrap the system and not replace it. The only place where it makes sense to replace a legacy system is when it has a high business benefit but is of low quality.
This was a question on my last exam, I felt it was worthwhile information.