Comment by Isognoviastoma

Current social consensus is that copyright exists and one can only use software on conditions stated in license. Thus, proprietary and copyleft have same protection.

Another possible consensus would be that copyright don't exist, and anyone can copy proprietary or copyleft work and improve it. Nobody would be harmed in such situation, original author still have its copy. I would have no problem with such state - but it must be same for everyone, not just FOSS.