"The irony with software is that the most successful pieces of software need to be changed frequently, because of changing requirements."

I don't think that this is true. Some software doesn't need to change much at all and yet remains quite useful. I'm thinking of common Unix utilities as a good example here. Some haven't changed much in decades and yet are still essential to many workflows.

I think what is closer to the truth is that many tasks that we write software to accomplish have rapidly changing or highly variable requirements. This is especially true of things that directly support business processes. Those kinds of software projects are either going to require a lot of changes or will need to constructed in a highly flexible manner so as to acomodate different needs.