Agreed, a sign of good programming is that the program feels easy and natural to extend. But there is a corollary, I think. A sign of good software development

Agreed, a sign of good programming is that the program feels easy and natural to extend. But there is a corollary, I think. A sign of good software development is that the program hasn't been extended in "unnatural" ways. That speaks to the developer's discipline and vision to create something that was fundamentally relevant to begin with.