The insurer is, at most at risk here, given the vulnerabilities of some systems and the lack of sense of security some individuals have.
Any risk is justifiable if the premiums are high enough. Any prospective policyholder's system would also be
thoroughly vetted by the underwriters. I believe that insurers will dip their toes in the water by further pooling the risk. For example, Coinbase's current policy for its hot wallet storage has multiple underwriters.
Regular people like you and me would never qualify for insurance like this. It's for large scale custodians or extremely high net-worth individuals who can afford the extremely high premiums.