I don't think that copyleft actually has any power over proprietary software. I don't see a legal end to users having a choice for proprietary software.
Currently, I see the copyright system as the only necessary legal core of copyleft. I'm open to learning other legal defenses of copyleft that don't depend on copyright at the core.
Is there another way to stop copyleft software from being used to create closed source software?
I imagine legal battles should currently be happening in this space, but I haven't heard of much, which is why I ask.