I don't think a fan blade failure would cause this. I also don't think a fan disk failure would cause it either as the fan disk is too far forward of the separation point. More likely is the failure emanated right at the point of separation of the fan module. Located at or very close to that that separation point is a shaft coupling (fan to LPT shaft) and a bearing structure. I think the conical fractured piece in the Ave Herald photo is part of the bearing support or seal system. More likely, there was a bearing problem that caused the shaft to fracture at that point. See the engine cross-section below:
https://www.tumblr.com/search/gp7200
Also, I don't think the remaining portion of the engine was about to fall off. The engine to pylon mounting system and attachment points are well engineered as are the pylon to wing mounting and attachment points. Think about something, during a turbulent air event, the plane could drop 500 feet or more and abruptly bottom out, the full weight of the engine (14,000 lbs + the weight of the nacelle) pulling against the pylon and pylon to wing. Vertical, side and rotational load factors are taken into account during the design processes with significant safety margins applied. Everything survives thanks to the robust designs employed...