Insurance companies actually have strong incentives to price things in a way that reflects the expected value of the money they'll have to pay out, and they have people who are paid to do those calculations. If some genetic variant makes someone ten cents per month more expensive, on average, then an insurance company would have trouble charging much more than that.
That's not really the threat, though. We're worried about what happens when someone has an 80% chance of having a disease that requires expensive treatment for the sufferer's whole life. At that point, insurance is either too expensive to buy or amounts to giving money away.
