The DNS is designed to provide resiliency to these kinds of problems by providing the ability to list multiple NS records located in different networks. It is standard practice for top-level domain operators and other high-activity domains to place their name servers in different networks to guard against these kinds of issues. When companies put all their name servers in the same network, they are removing the diversity benefit and create a single point of failure. Domain operators should take this as a cautionary tale that they shouldn't have all their eggs in one basket and make sure a single network failure couldn't take all their name servers offline.