What else do you foresee? A country like Chile just decides it doesn't want to be in the global internet and more, and separates? No, I think that's completely unrealistic.
If it can't upgrade in time, it might remain connected using some kind of translator or proxy. Even if not official, someone would surely run one - it's too useful and we're not talking about a censorship scenario where it would be illegal. Experience shows this is very annoying and will quickly be upgraded to native level. Note that tunneling is native.
Most end-user ISPs today use some kind of tunneling to separate the architecture of their network from the architecture of the services they deliver to customers. If you use DSL, your connection is (usually) a PPPoAoE tunnel with one endpoint at your house and another endpoint at one of your ISP's POPs - the entire access network feels transparent to you. If you use a cellular network, it does something similar with GTP.
And considering that fact, it's not as hard to upgrade a network to IPv6 as you might think. Some core routers and edge routers must be upgraded, but the majority of the network is tunneled over. Perhaps during a transitionary period, your CPE (home router) will encapsulate your IPv6 packets in IPv4. This doesn't require a new router because most of them do routing in software and can just get a firmware update.
If it can't upgrade in time, it might remain connected using some kind of translator or proxy. Even if not official, someone would surely run one - it's too useful and we're not talking about a censorship scenario where it would be illegal. Experience shows this is very annoying and will quickly be upgraded to native level. Note that tunneling is native.
Most end-user ISPs today use some kind of tunneling to separate the architecture of their network from the architecture of the services they deliver to customers. If you use DSL, your connection is (usually) a PPPoAoE tunnel with one endpoint at your house and another endpoint at one of your ISP's POPs - the entire access network feels transparent to you. If you use a cellular network, it does something similar with GTP.
And considering that fact, it's not as hard to upgrade a network to IPv6 as you might think. Some core routers and edge routers must be upgraded, but the majority of the network is tunneled over. Perhaps during a transitionary period, your CPE (home router) will encapsulate your IPv6 packets in IPv4. This doesn't require a new router because most of them do routing in software and can just get a firmware update.