I don't think that one unprovable assertion ("GPLv3 software has never killed anyone") is the best way to combat someone else's unprovable assertion ("our cars would be less secure if everyone could see our code"). The only way to argue against security through obscurity is to argue against security through obscurity, although admittedly that can be tough depending on the audience. Fighting against the status quo is just going to be an uphill battle sometimes, it's the nature of the beast.

This sounds more like a rhetorical device, in which debater #1 has attempted to frame the debate in terms of debater #1 being correct unless debater #2 can prove that they're wrong. Debater #2 then tries to reframe the terms of the debate such that debater #2 is right until proven wrong by debater #1.

In both cases it's more a form of sophistry vs. an opponent than it is a way to make a case to the public. Using a dishonest rhetorical device to counteract someone else's dishonest rhetorical device rubs me the wrong way, because it feels like both sides are trying to twist facts in their favor, and are not in good faith trying convince me that they're right and the other person is wrong. Debater #2 is responding to sophistry with more sophistry, rather than attacking debater #1's sophistry head-on.