If something is not legally available then copyright law ought to make it eligible for free distribution. The authors nor the rights owners certainly don't see the content making money anymore (or they'd be selling it) and thus they can't lose money (that they never meant to charge in the first place).
Copyright, as originally envisioned, should be a temporary privilege instead of a de facto ghost racket for perpetual extortion.
Old content can compete with new content, causing publishers to lose money. I'm not saying copyright law is good in the form that it exists today, but just because old content does not make money means that old content being available can't lose them money.
Copyright, as originally envisioned, should be a temporary privilege instead of a de facto ghost racket for perpetual extortion.