There's a difference between inaccessibility of the copyrighted material and non-existence of the copyright holder. In the last case it could go to public domain or become a state property. Or something else (any variants?). Of course such situation almost has no chances to occur in the modern industry, there publishers try to keep allied rights for themselves, but back in 80-90s there were a lot of independent companies, which sank without a trace and left their titles without an owner.
It's a professional curiosity
