This is all getting quite silly but really large sites with critical power needs (hospitals, factories, data centres, etc) will sometimes have two independent grid supplies running in from separate substations, preferably going as far back down the chain as possible before hitting a single point of failure. They'll also have UPS and diesel generators able to kick in and keep things running if somehow they entirely lose grid power. Its all totally doable, but as you say for most people going to the local library or a coffee shop is a perfectly fine fallback plan.