The conclusion of United Airlines Flight 232 was that although it was a manufacturing defect, the maintenance crew should have noticed the crack. Normally, cracks slowly spread over time until the part catastrophically fails, giving you time to catch it using various techniques (ultrasound, x-ray, penetrating dyes).
I'm not sure why you think it's reasonable to extrapolate the conclusion from an accident with a 1973 plane to an accident with modern hardware and vastly different fan materials. UA232 had a titanium alloy fan disk, while modern turbofans tend towards a nickel superalloy, sometimes even a single crystal.
