ROI

The core question

The whole thing in one story:

Your tap drips. Fix A reroutes the pipes for the whole house so that tap stops dripping. Fix B replaces the washer in that one tap. Both fix the drip. Only one of them means every future plumber has to understand the whole house's new plumbing just to work on a different tap someday.

That's it. That's the entire skill. It just asks, every time:

Are we rerouting the house, or replacing the washer?

• Replacing the washer — a local, contained fix at the exact leak.
• Rerouting the house — a shared boundary, new persisted state, a new lifecycle, or broad blast radius, paid to fix one drip.