Do they get rid of TLDs once the country they are assigned to goes away? I assumed they'd sell them to someone or something.

They might not have much of a choice but to shut down all .io domains. This video explains a little: https://www.youtube.com/watch?v=1yJ6AZsUlpc

Who is "they".

Nobody really knows. There are only a few precedents, i.e. the old Soviet Union .su tld being kept around, or the 2 letter country code that I can't recall which was reassigned to a new country after the old one went out of existence.

There isn't really a precedent for a tld with as many domains under it as .io, it's a very strong possibility it will be kept around and given to a private entity or even to GB.

.cs for Czechoslovakia lasted from 1990 to 1995.

.yu for Yugoslavia ran from 1989 to 2010.

Wikipedia has comprehensive articles on both of those ccTLDs, if you're interested in learning more

.su is still around.

That's why they are shutting .io to not make the same mistake again

Yes, it will be deleted; see: https://www.iana.org/help/cctld-retirement