It would be interesting to see a plot of price vs. uptime in the face of unlikely events for electric grids, and which spot on the curve has been chosen here.

There are definitely points on such curves, of course, where you've exhausted the sweet spot and just have the option of paying e.g. 3x as much for a very modest marginal decrease in risk. But that doesn't sound like the case here.

20% is about what energy deregulation supposedly saved customers. Utilities regulated under rate-of-return regulation tended to overbuild and maintain large safety margins, because they could pass those costs along to the customer.