If things get bad enough, users will simply point their resolvers to some alternate root. Trying to block DNS requests to alternate resolvers is a laughable proposition from a technical perspective.
ISPs can block outbound port 53 like they block port 25, or use DPI to detect DNS packets headed out of the ISP network. The only way around that is universal encryption, which the ISPs can slow down but not stop by blocking any high-entropy packets.
ISPs can block outbound port 53 like they block port 25, or use DPI to detect DNS packets headed out of the ISP network. The only way around that is universal encryption, which the ISPs can slow down but not stop by blocking any high-entropy packets.