It's also possible that the government may declare some bills invalid (what happened in India with larger bills). Also, government could inflate away your wealth by printing more currency.
At end of the day governments control both the network via (ISPs & Networking equipment) and processors (Try producing an 8080 in your backyard.).You make mistake of assuming that countries won't put up a global firewall that makes bitcoin/ether network impossible to operate. If China can filter traffic at internet scale, crippling Bitcoin network even if say EU/US/China are on board is trivial.
Currencies are backed by the only thing that matters, its will/trust of the people. And no amount of crypto changes that. Unless of course you have some magical way of baking silicon on your own into processors and other chips used in computers and networking equipment in your backyard.