This is an important point. Someone else is allowed to use the StackOverflow content and likely would if StackOverflow was turned off.
I do think that is worth noting, but it is (mostly) separate from the original point. It doesn't matter if the copyright is open if the content disappears.
At least in the U.S., I don't think it's that relevant to the use-cases I've seen. IANAL, but in the 4-factor fair use test a place like Internet Archive rehosting the content to me easily clears 3 of them if the original host goes away.
... which licenses its content with CC by SA 4.0[1] and has a public API.
Other licensing models of content will vary.
[1] https://meta.stackexchange.com/questions/333089/stack-exchan...