Complexity often enters one reasonable addition at a time.

Nobody sets out to make something complicated. It happens through a sequence of individually sensible additions, each one justified, each one small, none of them examined against the total. Six years later the thing is unmanageable and there is no single decision to blame. The mechanism is the accumulation, not any one step.

When adding something, look at the whole rather than the increment. Ask whether the total is still something you would design from scratch. If not, take something out before you add.