The GPL is a hack. If copyright goes away, you could just copy, disassemble, or duplicate the funcionality of a piece of sotware in order to learn how to tell your computer to do exactly what you wanted. DRM would go wild and hardware oriented, and a constant arms race would follow, but DRM may be an impossible problem.

For example, I'm producing a closed-source, proprietary piece of software to make a living. I will aggressively defend my copyright. Do I believe in copyright? No. But I'm operating in a bad system and I need to eat. In a better world, I'd be doing the same thing as work for hire for a group of actors within the industry who would collectively benefit from it, and customizing for any who had particularly unique use cases. I'd be working to reduce friction for the entire system and to make the tools that computers are generally more useful, rather than reenforcing the oligarchies of the market leaders who can pay my price (which is almost completely unrelated to my effort, but rather to the value of advantage over competitors in an underserved market.) My software being proprietary is just limiting general productivity gains and distorting the market (from whence comes it's value), and my enforcement is just limiting everyone's freedom to use their own machines.

another tl;dr, the GPL hack creates an island of freedom within a restrictive framework, and if the restrictive framework is gone there's no need to respect the GPL, a child of that framework.