FOSS depends on copyright laws. Oldest versions of Linux and UNIX are more than 20 years old, so early versions of both would be hypothetically now in public domain in the absence of Disney-lobbied maximalist versions of copyright laws, or in the total absence of one.
No it doesn't. Most projects are MIT or some other permissive liscense.
FOSS only "depends" on copyright laws in the sense that it is a hack to get around the existence of them (and now their children, anti-circumvention laws.)
And this would be a problem how? If copyright expired after 20 years and an evil commercial company privately forked the public domain Linux kernel 2.6.18, what would we lose?