I am currently reading the book computer science: an overview by J. Glenn Brookshear.And In chapter 7,there is one passage as follows:

In fact, it is often within this phase(modification) that a piece of software is discarded under the pretense (too often true) that it is easier to develop a new system from scratch than to modify the existing package successfully.

When I read this,an article by Joel occurred to me which mentions how Mozilla shouldn't have written its browser from scratch.(The article is here)

So,Is it mostly true that it's easier to develop a new system from scratch than to modify the existing one?Or it's closely related to the complexity of the system?