That's actually a great point. I feel like unless you know for sure that you will never need something again, nothing is disposable. I find myself diving into places I thought I would never care about again ALL the time.

Every single time I have vibe coded a project I cared about, letting the AI rip with mild code review and rigorous testing has bit me in the ass, without fail. It doesn't extend it in the taste that I want, things are clearly spiraling out of control, etc. Just satisfying some specs at the time of creation isn't enough. These things evolve, they're a living being.

For me, the answer to this question is: parts that involve no architectural decisions, and that won't need to be extended or built upon significantly in the future.

When I'm working on a greenfield project that I intend to build out further (which is what I am currently doing), I find that there's not a lot of work that fits those criteria. I expect that can change drastically when you're working on something that is either more mature, or more narrowly scoped (and thus won't need to be extended too much, meaning poor architectural decisions are not a big issue).