Open source was always around copyright because it was in spite of copyright not because of it, to fight against the notion of copyright using copyright itself as a tool, with copyleft as you mentioned. That is to say, the goals or values of the open source movements would be much better achieved if there weren't copyright in the first place, especially now that it's twisted to serve only big corporations and how books from over 50 years ago still aren't in the public domain due to Mickey Mouse laws for example. I am one of those few who do advocate for its abolishment.

There are also potential legal issues with public domain, especially in Europe where some rights can't always be disclaimed. There are OSI open source public domain licenses like the MIT-0 license.