Complex systems fail due to single causes exactly that way. First the system runs well; once the cause is introduced the system adapts so that the visible cost is minimal, but the adaptations are costly by themselves, and demand further adaptations; then the adaptation cycle runs for a while (that can be very long), until everything finally breaks down.
It is very unlikely that the lead pipes that were there since the beginning would be such a cause, but the duration of the fall alone is not enough evidence that it wasn't caused by an immediate problem.
It is very unlikely that the lead pipes that were there since the beginning would be such a cause, but the duration of the fall alone is not enough evidence that it wasn't caused by an immediate problem.