This makes zero sense. The companies that are currently publishing copyrighted proprietary software would then be free to take the GPL software, make improvements for their own purposes and distributed the compiled binaries and make profits without providing the improvements back.
As a proponent of permissive licenses I have no qualms about my software being enhanced and used by others without giving back those improvements - but the whole point of the GPL is to prevent that - so a GPL without copyright may as well be MPL or BSD or MIT.
Of course if I write a program I would like that others who make modifications (public) that they would also contribute those source codes too, but I do not like to put in legal obstructions, so you should have freedom rather than having too many laws and obstructions.
Without copyright the GPL (or any other license) is unenforceable