Without copyright the GPL (or any other license) is unenforceable
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.
I think it is OK to just not have copyrights (and also not have patents). And then, there is no non-free software licenses because it is not copyright, so even if they write them they do not have any legal meaning.
Stallman's idea, that practical works you can freely use/modify/distribute/etc but artistic works it is permitted to be a limited copyright (only a few years), might also work, I think (although I am not entirely sure; I think it is probably simpler to just abolish copyright entirely).
I have also seen a suggestion to tax copyright so that you have to pay a annual tax, and can copyright it for up to ten years, but anyone who pays 100 times the tax rate of copyright can force it into the public domain early. I think that can do (and if the government likes to earn money, it can help them too), but still perhaps the copyright should not be applicable for practical works, which instead are freely usable. (Some works may be hybrid, and that is more difficult. One idea is if you can separate the parts easily enough then you can copyright the artistic parts, but if you cannot then it is forced public domain; making the source code of a computer game available may be one way to do, so that you will want to release the source codes anyways in order that you may copyright the artistic non-practical-works of them.)
But whether or not is copyright, there should be a requirement of warning label on commercial products that use DRM, and there should not be laws to restrict circumventing them on your own copies of works/devices.