The trick is figuring out what this the priority. There is some 40 year old software that mostly work: it runs fast enough; it doesn't have many bugs; it doesn't follow best practices that the software world has discovered since; and it rarely needs new features. This code needs to be rewritten but the value just isn't there. There is other code that at only 5 years old needs to be rewritten: it is full of bugs, new features have been tacked onto an architecture that wasn't designed to support it - this software needs to have a large budget given to fixing it in the long run this will save a lot of money. Most projects are somewhere in between, you should give the team a little extra time/budget so that they can keep the maintenance up to do and it will never get much worse.