It's also worth mentioning that without copyright, all GPL-licensed code would enter the public domain, so you would then be free to modify it and only release the modified software as a compiled binary.
That's the plan (although RMS/FSF would also be happy with a less egregious copyright system; eg. like the early 14 year copyright system).
In the mean time, the copyleft is a clever hack which exploits the current copyright system to make it less biased against Free Software. If the copyright system goes, so does the bias, so there's no need for copyleft anymore.
