Literally, these are arbitrary strings following arbitrary rules. It's time to ditch ICANN and develop a parallel DNS that makes sense for today not the 90s.
Yes they can. They did it before after the Soviet Union broke up and they kept the .su TLD. It's still active. I'd argue that keeping around .io is more important than keeping .su around, seeing how many people and businesses use .io domains.
The Soviet Union ceased to exist. As long as the British Indian Ocean Territory is not breaking up or otherwise dissolving, it still is allocated a ccTLD.