But what concretely do they mean when they think that? I haven't seen many people argue that some law of physics prohibits level 5 autonomy, or that there's som

But what concretely do they mean when they think that? I haven't seen many people argue that some law of physics prohibits level 5 autonomy, or that there's some known plan which will take 100 years to execute. It seems to me that, when someone says level 5 autonomy isn't reachable in our lifetime, they're thinking something along the lines of "it's a really hard problem and I don't currently see a way to make progress".