GPL is a way to ensure that this particular piece of software stays free forever. One thing that GPL definitely doesn't do is "you have to contribute back your changes", it's fully legal to modify and use GPL-based systems and not contribute any changes back; the only thing that GPL prohibits is enforcing copyright restrictions on others when (if!) I pass the software on to them.

In a legislative regime where all software is free, that goal is achieved without GPL so GPL isn't needed anymore; I get the permission to use, modify and redistribute all software as if I had received it with the GPL.