Those liscenses, the GPL in particular, use copyright to fight copyright. The alternative to copyright is not necessarily a total free-for-all, you could legislate a replacement that works like the (A)GPL or CC BY(-SA). Require attribution and share-alike without restricting usage or distribution.
GPL uses copyright to ensure that software that makes use of GPL licensed code will be free (libre).
Even in a world without copyright protections, the printer story ( https://www.gnu.org/philosophy/rms-nyu-2001-transcript.txt ) would still have occurred, but there would be no way to ensure that it won't happen again with GPL licensed code.
Without copyright, open source code could still be locked up behind paywalls and compiled binaries. It is copyright that ensures that the law is there to force people to release their code.
The hypothetical legislation to ensure the GPL or CC-BY-SA works is copyright.
