> "What would the system’s structure need to be so that <some feature> would be no harder to implement than necessary?"

This sounds good, but in my direct experience it is really really hard.

For example, sometimes you have a feature that is really easy to add. Just add a new argument or keyword or command and implement it in the guts.

But every once in a while you get a beautiful architecture that has a "direction" to it. And a horrendous requirement comes along and breaks everything. For example, port it to macos. Or add and call this third-party library. Or break it up into an SDK, a CLI and a web service.

sigh. guess that's why this kind of career keeps you on your feet.