The instinct of an engineer under pressure is to add structure. Another layer, another abstraction, another file to make the system “more flexible”. Most of the time, what looks like flexibility is just optionality nobody will exercise — and the cost shows up later, when the codebase needs to be reasoned about, not extended.
Simplicity scales because it preserves the most important property of a system: that it can be understood. Tests, refactors, migrations and onboarding all run on top of that understanding. The moment a codebase stops being legible, every change becomes risky, and risk accumulates faster than any clever abstraction can absorb.
The discipline isn’t to avoid abstractions. It’s to keep them honest: introduce them when the duplication is real, the boundary is clear, and the alternative — keeping it explicit — is genuinely worse. Premature structure is technical debt that disguises itself as design.