You cannot "estimate" the probability of a catastrophic event, i.e. a Black Swan. All you can say is that it's possible, and over a long enough time period, things like that will happen. As well as things you never imagined.
No / smaller catastrophes: do the math, cash the premiums, do the payouts.
Big catastrophe: insurer would go under, but as long as it doesn't happen they can still cash the insurance premiums & everything is a-okay.