I mean smoothing out low-incidence, high-impact events is pretty much their business, no?
If they can know for a fact that a certain accident rate is guaranteed for a given car, then they will explicitly disclaim liability for accidents within that rate, similar to how all product warranties expire before the far end of the bathtub curve.
Otherwise it's like selling flood insurance on a flood plain, or health insurance to cancer patients: something that makes no economic sense and only exists where the government has forced it to.
"Exceptional case" is still a well-defined rate for them. They calculate risks and adjust insurance premium accordingly. That's pretty much their business.
With sufficiently large amount of insured parties those "exceptional events" are actually fairly common.
And that's what I was referring to. If the insurance company is in the business of calculating incident rates anyway they might as well cooperate with the autonomous car manufacturer to drive down incidence rates to a covered level. Just like car insurances will offer reduced premiums to experienced, accident-free drivers they can offer reduced fees to vehicle manufacturers who statistically cause fewer accidents.
The warranty example is bad because warranties end instead of getting renewed. They'd happily renew the insurance for the cost of a new product minus epsilon.