Last time I checked, their hot wallet had less than 5% of the bitcoins on their books. A catastrophic data break would leave most of their customers without either their bitcoins or any way to be made whole from the loss.

Right but it's only the hot wallet that has any significant risk. The cold wallets aren't online at all, they're on hardware wallets in geographically distributed safe deposit boxes. Even if someone broke into one and managed to crack the hardware, that'd be just a small portion of their funds.