Without copyright, arguably copyleft would be unnecessary, since the whole purpose of copyleft is to prevent non-Free code from being built on Free code, and without copyright, everything would be Free.
(OTOH, given the GPLv3, maybe that's not correct in the FSFs view, since the GPLv3 implies that "Free" has a broader and industry-dependent definition that merely eliminating copyright-based limitations wouldn't satisfy, so maybe the FSF does need to be copyright maximalists to advance their idea of Freedom-compelled-by-copyleft-license.)
Or no commercial entity would publish source.
I'm not sure you're knowledgeable enough on the subject to make that claim.
> You can argue over duration, but copyleft would not be possible if everything were in the public domain.
Copyleft wouldn't be necessary if everything were in the public domain.
If everything were in the public domain, nothing would stop you from forking an open-source project and distributing only binaries of your modifications. The precise difference between the GPL and say the MIT/BSD licenses is that the GPL precludes that. Copyright is the only hook you can use to compel creators of derivative works to distribute user-readable source.
Under MIT/BSD you can't, for example, use and distribute the changes to the Mac OS source code which were made after they forked from BSD.
There's still a bit of difference between GPL and just having everything public domain, I agree, but the difference isn't nearly as important as you're thinking it is.