free software is about freedom, not about licensing.
copyright is, by default, an obstacle to the essential freedoms, so we have to come up with licenses that relax the prohibitions so that others can have the freedoms they deserve. if copyright were to go away, that would remove one of the main obstacles for users and developers to have freedom.
it would also remove our main defense, namely copyleft. copyleft licensing is not a requirement of free software, it's a strategic choice based on available legal tools. if we were to lose that defense, publishing free software and hoping it remains free for all users would be a vain hope, because downstream copies, modified or not, could be separated from the essential freedoms. but the published version, accompanied by source code, would be freedom-respecting, thus free software, regardless, just like source code in the public domain today.