'You have to divulge your private key' meme

If they can update the firmware then they can still make a good faith effort and resolve the issue. If they can't do that for some contrived reason then I guess the copyright holder, whose software the vendor is distributing without a license, could sue for an injunction against further sales and ask for some kind of relief. The GPLv3 is on pretty solid legal ground so I really don't see it coming to that as a practical matter, I'm not sure what vendor would willing to back themselves into a corner that way. It should go without saying that the license doesn't need to protect vendors who are acting in bad faith from their customers and suppliers.