"Governments across the world have been concerned that the US government could theoretically knock their individual internet registries offline."
The US has had this theoretical power for the last 30 years. They've never used it. Even if they did, it wouldn't stop dotted IP addresses from working and wouldn't stop people setting up alternative DNS nameservers that continued to dish up names for the blocked TLDs.
During that period they have invaded several countries and rattled sabres with several nuclear powers, so I think we can conclude that their reluctance to pull the plug on DNS is not simply a case of "We haven't been angry enough, yet.".